I'll always encourage the game gradually gaining classes and continuing to grow. In terms of what some of these might be like a Tinker or Dragonsworn would be cool as some have said. I also really love the concept of playing a Necromancer and believe it can differ easily from Death Knights and Warlocks. Here are some of my ideas for them:
Tinker
Tinkers could be a versatile class with many options, something that themes in well with their inquisitiveness and desire to always learn new things. Sources of inspiration to draw on would be the unusual-effect heavy, nuts-and-bolts and energy style of Gnomish Engineering and the more practical and explosive/chemical nature of Goblin Engineering. Other possible sources would be the Forsaken's Mad Scientist-like Alchemy, Blood Elven Technomancy, Draenei Crystal Technology, Dwarven Metalworking, and possibly other stuff like Worgen Gun Smithing. They're are several races besides just Goblins and Gnomes that have skilled engineers with their own styles of craftsmanship.
In terms of possible specs, like De Lupe said on page two a tanking spec where the Tinker summons a mechanical suit or vehicle would do well as they're are a lot that things like durable steel plating, force fields, regenerative nano-tech, and steam technology could bring to it's identity. An explosive-based DPS spec could also be very cool because not only would it have potential for strong AoE damage but it could also have a supportive role through its turrets and cannons which would be especially helpful in PVP when it comes to protecting an objective. A Chemist healing spec would be able to not only possibly provide new options for sustained HoT and AoE heals among other things but also be able to fend well for itself in a support and damage role through the use of buff-granting potions, poisonous and burning gas clouds from thrown/spread chemicals that debilitate enemies, teleportation tech and smoke screens to save allies and other ideas.
It could also incorporate elements like collecting scrap (think Gazlowe from HotS), finding new upgrades through the use of talents, and other elements to help give the class more identity and feel to it.
Dragonsworn
The dragons of Azeroth are an important part of the world's lore and possess several different powers among their flights that give them plenty of inspiration to draw from. With the dawning of the Age of Mortals at the end of Cataclysm and groups like the Timewalkers coming to be, it makes sense that dragons would begin to train mortal champions as Dragonsworn and teach them to use their remaining abilities to assist them in protecting the world. Each of the flights have a unique element they can bring to the class: The Red Flight's burning flames of life that also heal, the Green Flight's connection to nature both to heal and to poison and debilitate, the Blue Flight's icy breath and arcane magic, the Bronze Flight's sand attacks and time-distorting powers, and the Black Flight's raw channeled fury through lava and earth-based attacks and defensive abilities. Placing these powers into three specs it could be something like:
Tank-style powers: Black, possibly Red or Bronze
DPS-style powers: Blue, Bronze, Black, possibly Red or Green
Healing and Support-style powers: Red, Green, Bronze, possibly with mana and spell assistance from Blue or physical buffing from Black
Having a class that incorporates all these different powers might seem challenging at first, but not if you consider all the flights hosting a meeting at Wyrmrest and deciding to form a unified group to teach their combined might to the champions. To help distinguish them and give each one more meaning, a unique class mechanic could be implemented where you could have different categories of abilities to draw and learn from for each flight in some way and different talents and training could offer unique versions of abilities perhaps.
As for just really cool abilities I can see something that lets you temporarily assume a Dragon-like form to be cool for either travel or a combat skill, or maybe talents that call upon the age and ancient strength of the proto-dragons or former Aspects in some way.
Necromancer
When it comes to these summoners of the dead one of the most iconic images is the bearded and skull-wearing Scourge necromancer, but they're are other cultures that draw upon the dead. The Shadowmoon of alternate Draenor use the power of the Void to summon their dead for example, and the warlocks of Warcraft 2 created Death Knights through a form of spiritual transfer using the power of sacrificed Necrolytes, the Mogu bind spirits into terracotta warriors, and some Trolls practice a form of Voodoo necromancy (like the Sandfury, Atal'ai, and Shadowpine.)
In terms of how they could be different from similar classes, as Sakk mentioned on page four Blizzard could draw upon elements from the Diablo 2 Necromancer class which was phenomenally well-done. Hallmarks of the class could be weaker but more numerous minions compared to the pets of Warlocks and Death Knights, having the spell to raise them needing or receiving bonuses from casting them upon dead mobs and players, a variety of attacks and support skills themed around bone, poison, blood, spirits, scythes, curses, and diseases. Being able to summon a stronger minion to use as a more singular pet similar to the Diablo 3 Witch Doctor's Gargantuan, and generally making use of a wider array of types of dead you can summon from the Scourge's catalog of undead minions. It could range from weaker ones like zombies (good for sacrificing in shambling charges) and skeletons (with different models depending on the creature you raised it from, possibly via glyph as a cool touch. You could also summon skeleton warriors, archers, and mages for variation among your attack force) to ghouls, abominations, worms and carrion insects, geists, ghosts and revenants, banshees, bone constructs, and eventually Frostwyrms.
Another really awesome ability would be allowing yourself to become a Lich in some way. I know it's more of a one-way thing because you've got to die to become one and you can't change back, but they could still make it work through some thematic means I'm sure. Transformation into a Lich could also allow special access to more frost-based powers because control over the frigid winds of Northrend was a gift granted to them by Ner'zhul and Arthas in the lore.
In terms of role I could see them primarily being a DPS class similar to Mages and Warlocks with some Warlock-style healing elements, using its army of mostly disposable minions and debilitating and imprisoning spells to stop an enemy. The themes of each spec could be like this:
Blood
Similar to the Death Knight spec, this spec uses blood and bone to attack an enemy. This spec allows Necromancers to drain blood similar to the San'layn and possibly use it as a type of resource, and shape bone into weapons and obstacles like spears and cages. Preferred minions are of the fleshy and skeletal variety, granting new bonuses to basic skeletons and zombies, and allowing access to tougher reanimations like abominations and constructs. These minions are capable of tanking for the Necromancer and assuming different formations to support them on the field of battle. Skeletons could carry out their different roles from multiple areas around the Necromancer and allow for more diverse and strategic gameplay, and the fleshy minions could be used to take the brunt of attacks or be sent in as living bombs to be exploded by the Necromancer.
Spirit
This spec takes a more ethereal approach to combat, and focuses on draining the enemy's soul away and using the Necromancer's spiritual ties to the Shadowlands to summon ghostly apparitions to aid them. While not as durable as the minions summoned by Blood, Spirit's minions have a diverse array of sneaky abilities that could slow the enemy down. It relies on less direct and more magic-based attacks in general, cursing the opponent and haunting them with apparitions.
Poison/Disease
This spec is a little similar to an Affliction Warlock and uses a more DoT-filled attack-style, but has it's own unique theme patterned around poison and infectious diseases instead of curses. It's focused around the ickier side of undeath, conjuring noxious clouds and corrupting the life in an area like how the poisonous grain did to Stratholme. They summon more slimy and diseased minions and enchant them similar to the Diablo 3 Witch Doctor by setting them ablaze or covering them in rotting pustules.