In this thread, i'm going to discuss the possible future race/class combinations, based on lore and other sources:
Demon Hunter
Orc
Gul'dan, already, had demonic spikes sprouting from his back.
Demon Spikes
Instant 20 sec recharge
2 Charges
Requires Demon Hunter (Vengeance)
Requires level 1
Surge with fel power, increasing your Armor by (75 * Agility / 100)
Vengeance (Level 41)
and your Parry chance by 15%, for 6 sec
Siege of Orgrimmar had Blind Blademasters (notice the blindfold and body tattoos):
I don't see a reason why there couldn't be an Orcish version of this:
Gul'dan, already, had something similar ,during the Nighthold raid:
(Notice the spikes, rocky outcrops, horns and flames.)
Much like how Illidan consumed the Skull of Gul'dan (a demonic artifact) to empower himself and become a demon, Orcs drank the Blood of Mannoroth to empower themselves and become demonically corrupted:
Yet, they hate the demons as much as the Illidari, for corrupting their race and destroying their world. Orcs are no strangers to demon-slaying:
Nightborne
Much like Illidan, they are former Highborne.
Felborne, already, have units that look and behave very much like a Demon Hunter:
Felborne Marauder/Felsworn Noble: Fel Streak, Felblade Slash.
Felborne Spellbreaker: Burning slice, Fel blade.
Duskwatch Felblade: Fel Strikes.
Duskwatch Marauder: Demonic Slice, Fel Charge.
Felsworn Overseer: Blast Nova.
In Hearthstone, A Nightborne is depicted as a Demon Hunter:
Void elf
Much like the Nightborne and Night elves, Void elves are former Blood elves. It can be assumed that like their Blood elven brethren, they can, potentially, be Demon Hunters.
Void elves have Warlocks among them, called Rift Wardens (previously called Void Warlocks). So, it is not so far-fetched to see them being associated with fel and demonic magics.
In Hearthstone, a Void elf Demon Hunter is depicted:
Troll
"Troll warlocks date back to the Zandalari demoniacs, who seek to embody power outright and not just possess it. While warlocks tread dangerous ground in dealing with demons, demoniacs take it one step further. They allow demons to enter their bodies through an ancient and arcane ritual. Rather than let demons possess them, the demoniacs enslave the demons from the inside, absorbing the power of the beast for their own command. It is a feat very few are able to master and more than one has lost the fight, but the power they are able to unleash is worth the sacrifice."
In Hearthstone, a Troll Demon Hunter is depicted:
Zandalari Troll (Reservations)
While it is said that Troll Demoniacs originated from the Zandalari Trolls, Blizzard seems to have retconned it with the addition of playable Zandalari:
"In Mists of Pandaria, demoniacs were one of the only professions not to appear among the Zandalari, contrary to haruspices and others, and in Battle for Azeroth, Zandalari have a much more holy aspect to them. Following the announcement of the Zandalari allied race, Blizzard stated that Zandalari have no connection to fel magic and therefore cannot play as warlocks. So it is likely that Zandalari abandoned this practice a long time ago."
Pandaren (Reservations)
Master Cheng is a pandaren monk dreaming of being a demon hunter instead:
"Yes, I am a monk, though I always dreamed of being a demon hunter."
Even so, associating Pandaren with fel and demonic magics would present a problem:
"During the development of Mists of Pandaria and Pearl of Pandaria, the pandaren were originally depicted with green eyes, but Blizzard artists later learned that green eyes are believed to be a sign of demonic corruption among certain Asian cultures. In the interest of being sensitive to those cultures, the pandaren's eyes were changed to gold."
Draenei
Never thought i would associate them with Demon Hunters, due to their Holy themes and lack of a Warlock class. Yet, a Draenei Demon Hunter appears in Hearthstone:
I can only attribute that to their goal of fighting the Burning Legion:
"The draenei's steadfast resilience against the Legion is the definition of perseverance" (Heroic Presence).
Human
In the RPG sources, they can be Demon Hunters.
Kanrethad Ebonlocke, a Human Warlock, transforms into a Demon in a similar manner to a Demon Hunter:
Forsaken
Much like Troll Demoniacs, there is a single case of an Undead absorbing a demon into his body, called Meryl Felstorm:
"In order to lure the demon away from Valeera's body, he challeged the demon to possess his body instead and being the stronger of the two wills managed to repress the demon in his body. Recognizing that his demonic possession was a liability to those around him, he declined membership in the New Council of Tirisfal and left in self-imposed exile, renaming himself Meryl Felstorm as a sign of his corruption".
(Kor'vas Bloodthorn: "They gnaw at our every thought")
Dwarf
Apparently, there is a Dwarf Demon Hunter in Hearthstone:
Druid
Orc
Conversation
Rala Wildheart says: The situation has become very dangerous on Ashran lately. This morning I nearly had my head taken off by an enraged ancient.
Shadow Hunter Askia says: Da Alliance must be desparate to obtain da artifact to unleash such foes upon us.
Rala Wildheart says: The fury of nature should not be abused in such a way. In the end, they will pay for their folly.
Rala Wildheart says: The primal forces of Draenor are wild and untamed. They seem to resist any attempt at control.
Shadow Hunter Askia says: Dat must make things difficult for ya on da battlefield.
Rala Wildheart says: Not necessarily. I still have my teeth and claws.
"Rala's appearance, as well as her name and conversation, seems to depict her as a druid. Orc druids are not currently an allowed race/class combination; Rala may, therefore, be a foreshadowing of the combination becoming playable in the future."
Much like the Worgen, the Orc clans had a connection to Goldrinn, who revealed himself to them on Draenor. This can be seen by their connection to wolves:
"Lo'Gosh has always been with the orcs on Draenor as a part of their Horde, in their instincts and blood. When the orcs arrived on Kalimdor, they were told of Lo'Gosh by the tauren. The orcs adopted him as their hero on this new world, for they considered that the will of orcs is strong like his and, like him, not even death can stop them."
Orcs, also, have a deep connection to wildlife:
"For reasons unknown, orcs also share a great natural affinity with animals. Many animal-orc friendships are made as soon as they look into one another's eyes and find the same primal soul within each other. Their close mental connection to these beasts has led them to be able to [Command] beasts with ease".
In the RPG sources, Orcs had the Primal class, which is similar to a Druid in many ways:
"Primals are those warriors who revel in their bestial rage and give into it completely. As they slowly take on an increasingly beast-like appearance, they also gain powerful and violent abilities. Primals believe that every creature is inhabited by a dark, primeval essence. They hold to the notion that each and every living soul is haunted by a beastly force, and that those who accept this fact can eventually ascend to another level of existence - one that is more primitive, but also more connected to nature and the animal kingdom. Though very few can fathom the existence of this whimsical force, primeval characters strive to unlock its secrets. They call this pristine essence "the beast within". Primals believe that once someone has accepted the beast within himself, he must learn not to suppress it — as most civilized folks were brought up to do. Indeed, the character must welcome it into his heart and soul in order to call upon it in times of need. Thus, primals can tap this essence and channel the instincts, strength, and fury only the beast within can provide."
Their animalistic behaviour can be seen by clans, such as the Frostwolf or the Thunderlord.
Geyah to Durotan, during Lords of War: "I would rather have died, than know my son had become a beast".
Maraad on the frostwolf clan, during Lords of War: "Those who succumb to the beast within, become their own greatest enemy".
Animalistic armor:
Mag'har Orc
Everything that applied to Orcs, applies to the Mag'har Orc, as well. Even so, i want to point out one aspect of the Shadowmoon clan - astromancy:
"The Shadowmoon had a history of astrology, they had always spoken the language of the stars, the earth, and the moon."
Blood elf
Blood elves have rangers among them, who practice a minor form of nature magic:
"Elven rangers are elite archers turning to nature for aid, befriending animals. Some of them possess minor druidic abilities, as Vereesa Windrunner was able to use the "language of the trees, of all plants", and sense a plant's emotions and history through touch."
High Botanist Freywinn can transform himself into a treant:
Nightborne
Much like the Shadowmoon clan of Orcs, Nightborne have several astrologers among them:
And, much like High Botanist Freywinn, they have a botanist of their own who can transform into a Botani (High Botanist Te'larn):
Dwarf
The Wildhammer clan is known to have Druids in the RPG sources.
There appears to be a female dwarf using druidic magic in the card art for Small Repairs in Hearthstone:
Much like the Tauren, they are known to wear feathers, and have bird totems:
Pandaren
The enigmatic pandaren are one of the most elusive races of Azeroth. Hailing from Pandaria and the Wandering Isle, they are humanoid pandas with a great love of nature and strong ales.
Much like other Druidic races, they have a connection to Wild Gods, called the August Celestials:
"When they first settled in the land later known as Pandaria, the pandaren formed a system of worship centered around four Wild Gods that they called the "August Celestials", believing the powerful beings to be benevolent deities. After Lei Shen defeated the Celestials during his conquest of Pandaria, worship of the animal spirits was outlawed on pain of death. Many of the pandaren's ties to the Wild Gods were lost, but a few brave individuals secretly continued preserving their teachings."
Gnome
Hearthstone depicts a Gnome Druid:
Which, doesn't make much sense as they often push nature aside to focus on their love for technology.
Mage
Tauren
In the RPG sources, they can be Ley Walkers, who are a type of a mage:
"The ley walker is a magician who has discovered this fact and seeks to harness the power of these threads. A ley walker is aware that, every moment of every day, these threads permeate all things, passing through, around, above and below all things, carrying magical energy from the Well of Eternity to all things in the world. With diligent study and practice, ley walkers sense these powers."
Monk
Goblin
A Goblin Monk exists, named Gakkiz Blusterblast:
Worgen
I don't see a reason to add Goblins, but leave out Worgen. Every race, except for them, can be taught the ways of the Monk through Pandaren teachings.
Lightforged Draenei
Same as with the Worgen. No reason everyone would get it, but they won't. Besides, Lightforged Draenei are just Draenei infused with the Light. And the Draenei can already be Monks.
Paladin
Gnome
In Hearthstone, there is a Gnome Paladin:
Night elf
Several Night elf Paladin NPCs exist, countering the Tauren Sunwalkers' belief in the Sun with that of the Moon. Delas Moonfang, a former Priestess of the Moon, turned into a Paladin in Legion and joined their class order hall:
"After a fateful encounter on a Legion world, she converted to a paladin of the Knights of the Silver Hand, saying she had always felt her education incomplete. She is the first night elf to join the Order of the Silver Hand."
Nerus Moonfang:
He uses abilities like Holy Flare, Blast of Light, Crusader Strike, Divine Storm.
Alynblaze:
He uses abilities like Blessing of Might, Divine Storm, Hammer of Wrath, Retribution Aura.
Eternal Champion:
They use abilities like Queen's Blade and Shimmering Strike.
Worgen
In Hearthstone, a Worgen Paladin is depicted:
It makes some sense, as the Kingdom of Gilneas, likely, had many Knights and the belief in the Light is, already, a part of their culture.
Troll
Much like their Zandalari cousins, the Darkspear Trolls can mimic the Paladin through their Loa worship, most notably Shirvallah:
Forsaken
Even though it causes them great pain, Forsaken are able to wield the light:
"When undead channel the Light, they do not disintegrate or explode from channeling the Light, though they may wish they would. Instead, it feels to them as if their entire bodies are being consumed in righteous fire. Forsaken healed by the Light (whether the healer is Forsaken or not) are effectively cauterized by the effect: the wound is healed, but the healing effect is cripplingly painful. Thus, Forsaken priests are beings of unwavering willpower.
Despite all this, there exists at least one account of someone becoming undead through the power of the Light: Calia Menethil."
There's a Forsaken Paladin depicted in Hearthstone:
Highmountain Tauren
"Much like the Kalimdor tauren, the Highmountain tauren worship the Earth Mother and her two eyes, An'she (the sun) and Mu'sha (the White Lady)".
There's a zone in Highmountain called the Fields of An'she.
Apata Highmountain yells: "The beast is there! An'she protect us!"
Kul Tiran
Much like Gilneans, they had Knights among them and had a belief in the Holy Light.
Even though this faith seems to have been replaced with sea-worshipping, Hearthstone depicts a Kul Tiran using the light:
In fact, during Warcraft 3, Kul Tiras employed Paladins, Crusaders, Knights and, of course, Chaplains into their ranks.
Pandaren (Reservations)
"Pandaren priests, what they believe in and from where they draw their powers have not yet been addressed by canon sources. However, some in-game references do exist. Omnia Priests provide several quotes which refer to the 'light':
Let the light protect us!
Our enemies underestimate the power of the light.
The light will prevail.
The leader of the Omnia discipline, the priest Yalia Sagewhisper, also mentions the light in several of her quotes:
May the light guide you.
Be blessed by the light.
Light is right.
Even the light can run out of patience!
The above may be taken to suggest that pandaren priests do indeed wield the power of the Holy Light. The Omnia Priest quote "Shadows, begone!" and Yalia Sagewhisper's quote "Keep the shadows at bay" may be taken to further suggest that the pandaren share an experience of priestly ways (specifically the division between the Light and the Shadow) similar to races such as humans.
One contradiction to the above is provided by Jojo Ironbrow when first hearing of the term "the Light", responding "What's the Light?" However, this may be put down to simple (if surprising) ignorance on Jojo's part, or to a minor error by Blizzard. One other possibility would be that the term is not used on the Wandering Isle, Jojo's home, but is used widely among pandaren priests on Pandaria itself. Pandaren priests on the Wandering Isle may then share the same beliefs as their Pandarian brethren and simply use different terms, or may have more substantially differing ways."
I believe, therefore, that if Pandaren would get Paladins, that they would draw their powers from the August Celestials, much like how Trolls draw theirs from the Loa (both, being Wild Gods):
Temple of the White Tiger
Temple of the Jade Serpent
Temple of the Red Crane
Temple of the Black Ox
Shrine of Bwonsamdi
Shrine of Krag'wa
Shrine of Akunda
Shrine of Gonk
Shrine of Pa'ku
Shrine of Kimbul
Priest
Orc
Much like the alternate universe Mag'har Orcs, the main universe Shadowmoon clan had connections to the Dark Star, according to Chronicles: Volume 2.
Unlike their alternate universe equivalents, they didn't resume this practice at the behest of Grommash Hellscream.
"A few bold shaman tried to access K'ara's shadowy power, but the Void energies shattered their minds. When a clan member was found defiling the spirits of the ancestors, the chieftain declared that the Dark Star and its magic were evil and dangerous and would lead the clan down a path from which they could not return. It was decreed that the Dark Star's powers of shadow were forbidden to the Shadowmoon. Anyone who tried to wield them was sharply rebuked, and if they continued they would be immediately exiled far from Shadowmoon Valley."
They could, possibly, resume that practice and embrace it.
"Ogath the Mad is an orc quest giver located in Spinebreaker Post in the Hellfire Peninsula. Seeking to command the powers of the void he sends the adventurer to retrieve a void creature's soul shard for him.":
Highmountain Tauren
Same case as with the Paladin class. The fact that they worship An'she could, potentially, introduce them to the Priest class (Seer).
Rogue
Draenei
Rogue - The Rangari employ rogues in their ranks.
Altauur
Tauren
In The Shattering: Prelude to Cataclysm, there were Grimtotem assassins attacking Thunder Bluff, as well as technicians that set out bombs. There are also a number of Grimtotem mobs that use Ability [Stealth] and other rogue-like abilities, indicating that the infamous tauren rogues can be found amongs their ranks.
"Those led by Jevan Grimtotem, who are loyal to the Horde and renounced Magatha after the part she played in Cairne Bloodhoof's death. Jevan's Horde Grimtotem are about a quarter of the pre-split Grimtotem population."
Grimtotem Tauren are a possible future customization option for Tauren, as can be seen by Torkhan in Orgrimmar:
Shaman
Human
"Highlord Demitrian, a servant of Thunderaan, has a connection with the elements unusual in a human, suggesting that he may be a rare example of a human shaman. The non-canon Warcraft RPG explicitly states Demitrian is a shaman.":
Blood elf
"With the Twilight's Hammer getting busy, many races of Azeroth joined their cause. One blood elf is known to have become an elementalist shaman.":
Nightborne (Reservations)
There are Nightborne Astral Farseers in the Nighthold, but they are more connected to the stars than to the elements. Though, Shadowmoon Orcs are astrologers, as well, and they are known to have shamans among them.
Gnome
Like the Goblin shamans, they could use their technology to call on the elements:
"As for the goblins' "mechanical" totems, note that these are merely physical manifestations of the small totems they tinker/craft to form a link with the elemental spirits. Instead of lugging around large totems, goblin shaman have a ring (probably the same ring on which they keep their house and motorbike keys) with small totems they've built as conduits for the elemental spirits they do business with."
"Rather than using traditional wooden totems to channel the spirits' power, goblin shaman infuse their technology with elemental energies, using mechanical totems instead."
"There are many ways to commune with the elements. In their ingenuity, the goblins have found ways to imbue their mechanical devices with the energy of elemental spirits."
Mechagnome
Like i mentioned above, they could use their technology to employ the elemental spirits. This is what Prince Erazmin has to say about them:
"Every shaman I've ever met has a favorite element... even if they won't admit it.
You require more lightning? Stuff your arms with a variety of metal gadgets to make them perfect, efficient conduits of that electricity!
More interested in lava? Perhaps try internal forges that'll keep you fueled up and ready to unleash molten rock at anything that moves.
Water is easy. Most people are made of the compound. We've created pumps that tap right into that personal reservoir when you need it most!"
As for the concern that using water magic might cause their robotic body parts to begin rusting, Dark Iron Dwarves can use it, despite being infused with fire.
Warlock
Night elf
In the RPG sources, Night elves can be warlocks.
The Shen'dralar Highborne are the most likely, as they've dealt with demons in the past:
"Tortheldrin eventually formulated a plan to revitalize the Shen'dralar. He forged pylons in the halls of the west wing of Eldre'Thalas, constructing a prison to house a new source of power behind a grand force field: a demon named Immol'thar. To the shock of the other Shen'dralar, Tortheldrin had covertly summoned and bound the terrifying creature to siphon the demon's power and give it to his followers. Any objections were quickly settled once the other Shen'dralar experienced the demon's energy for themselves. Though dark and volatile, Immol'thar's power was invigorating and addictive, more so than the Well of Eternity had been. The siphoned energies would sustain the Highborne's magic, allowing those within the walls of Eldre'Thalas to feed their endless and growing magical pangs."
Draenei
Draenei Warlocks exist, like Doomsayer Jurim:
He uses abilities like Corruption, Incinerate and Fear.
One could claim that Blizzard just mixed up Eredar with Draenei. But, they had a unique model for an Eredar back then that was based on Archimonde and was separate from the Draenei model. So, they could've used it:
Lore-wise, it doesn't make much sense, since they rejected Sargeras' gift and split off from the Eredar to follow the Light. But, i guess if they are eligible for Demon Hunters, Warlocks make sense.
Zandalari Troll (Reservations)
While it is said that Troll Demoniacs originated from the Zandalari Trolls, Blizzard seem to have retconned it with the addition of playable Zandalari:
"In Mists of Pandaria, demoniacs were one of the only professions not to appear among the Zandalari, contrary to haruspices and others, and in Battle for Azeroth, Zandalari have a much more holy aspect to them. Following the announcement of the Zandalari allied race, Blizzard stated that Zandalari have no connection to fel magic and therefore cannot play as warlocks. So it is likely that Zandalari abandoned this practice a long time ago."
Highmountain Tauren (Reservations)
The Bloodtotem tribe, who would come to be known as the Feltotem tribe, had Warlocks among them due to their pact with the Legion, like Shara Felbreath:
She uses abilities like Fel Implosion, Shadow Bolt and Summon Imp.
"Following the end of the third invasion of the Burning Legion, survivors of the Bloodtotem tribe who didn't choose to join the Legion settled at Thunder Totem."
Since those that joined back with the rest of Highmountain didn't join the Legion, i don't know how much this possibility is viable.
Mag'har Orc (Reservations)
"Necrolytes are a type of necromancer who use the dark energies of the Void, as it was discovered by Natalie Seline. As such, they sometimes overlap with warlocks and shadow priests in their depiction."
"In the alternate Draenor the Shadowmoon clan is also depicted as having several masters of the necromantic arts, including necrolytes, a reference to their original description as "practitioners of the Orcish religions"."
Since Necrolytes have little to do with the Fel, but rather with Void and Necromancy, they could avoid treading on Fel corruption. Just like how Void elves can be Holy Priests and Lightforged Draenei can be Shadow Priests for gameplay reasons only.
Though, practicing Fel magic would turn their skin green, and therefore corrupted (non-Mag'har):
The Mag'har orcs ("uncorrupted" in Orcish), also known as "brown orcs", are the orcs that escaped the corruption of the Burning Legion and thereby retained their natural brown or gray skin color."
Their Racial Trait wouldn't hold true anymore:
Savage Blood
Mag'har orc racial
Passive
Reduces the duration of Poisons, Diseases, and Curses by 10%.