SEAS OF NIGHTMARE (total speculation)
Context:
The Keepers are all dead, Odyn has been told by Jaina to use the Pillars of Creation to seal the Legion's portal at the Tomb Of Sargeras in case of failure on our campaign on Argus. The Portal's explosion opened the titanic prison beneath the TOS and freed a massive amount of corrupted Souls and N'Zoth. Odyn went crazy and traveled to Ulduar, where he tried to corrupt or kill his former mates, the Titanic Keepers, and use their power to resurect Yog'Sarron.
Heroes went back victorious from Argus, but found a bigger threat in Northrend, so they've been sent to stop the Prime Keeper.
In order to stop the resurection, they had to sacrifice their legendary weapons to seal Ulduar. Sylvanas succesfully enslaved Odyn's valkyrs after the Prime Keeper's death, Nobundo took the Draeneis' leadership since Velen doesn't need to protect his people from Kil'Jaeden's threat anymore.
N'Zoth has escaped from its prison and Queen Azshara finished the construction of Nazjatar.
The Old God has settled in the center of the Nagas' island, in a building made for him: the Tower Of Sacrifices. The Nagas made an offer to the Bloodsail Buccaneers who live on the Plunder Isle: if they join the Old God's side, they'll have their revenge on KulTiras and Booty Bay. N'zoth asks Nagas and Bloodsail Buccaneers to raid over the nearby islands and capture the local populations and bring them to Nazjatar, in order to sacrifice them.
New Map: Lost Islands
Legend:
Red Arrows: Raid Assaults
Blue Arrows: Escape fleets
Note: could contain one or two more islands related to Yaungols and other alternative races.
New feature: Alternative Races
Hord:
Goblins -> Gilblins : Before the Cataclysm happened, Hobart Grapplehammer created the GilGoblins, a mutated race of Goblins able to live under the water, but the experiment went wrong. This race was primitive and aggressive, so when Deathwing attacked Kezan it was a relief for the Goblin mad engineer to leave the place. Nagas came to enslave the GilGoblins and use them in Vash'jr and Azsuna, but some of the mutated Goblins stayed near Kezan's shores. When the Volcano in the center of the island stop to spit its lava, the GilGoblins went back on the ground and found the Kajite's mines. The minerals changed them and made them more calm and intelligent. They named themselves the Gilbins, and reinvested the former Goblin's capital, until the Zandalar Trolls came to offer them to travel to Orgrimmar because of Nagas' threat.
Taurens -> Yaungols: When the great Sunderring happened, some Yaungol tribes been trapped on islands.
Orcs -> Mok'nathal: During the first War, the Hord used the Mok'natar clan as cannon fodder against the human forces. When the Hord has been defeated, the half ogre / half orc clan decided to flee with some ogres accross the sea. They found an island full of vegetation and settled there to live a peaceful and restful life.
Blood Elves -> Wretched After the freedom of Suramar and the Legion's defeat, the Nightwell was slowly dying, leaving most of the surrounding city empty. Most Nightbornes tried to join Shal'Aran to use the Arcandor, but it created some tensions between Suramar refugees. Tyrande asked First Arcanist Thalyssra to join her in Darnassus, but only if she kept the healed Nightbornes. When Thalyssra left, the refugees stayed at Shal'Aran, but the Arcandor wasn't enough to heal them completly. Seeing this, Lady Liadrin asked Lorthemar Theron if the Blood Elves could accept the Wretched and cure them with the Sunwell.
Forsaken -> Reincarnated: Since Sylvanas got Odyn's valkyrs back, she experimented the same process she used on Nathanos Blightcaller, using human prisonners as vessels for her forsaken. This is the only way to keep her people alive and sit her power for eternity... event if it takes many battles to win.
Darkspear Trolls -> Zandalar : Zandalar Trolls used to sail over seas after the Great Sundering, for the sake of their former glory, some of them tried to resurect the Thunder God, Lei Shen, but got defeated. Meanwhile, other Zandalar Trolls chose to stay far from these old dreams and continued their live on Zandalar Island. With years, the Nagas' threat became bigger, and they are now powerless since their island hold a secret power convoited by the Old God's minions. A new but familiar Loa spoke to them, convincing them to sail to Orgrimmar and ask the Hord for some help. Vol'Jin foresaw that Zandalari could join the Hord for their own survival.
Alliance:
Humans -> Azothas: These humans were siblings of Vrykuls that went south of Eastern Kingdoms, surviving in Trolls' territories. They call themselves Azothas, and they differs in many points from the Eastern Kingdoms' humans: they're tall and strongs, adopted druidic and shamanic cultures from Trolls and kept the Vrykuls' knowledge about sailing.
Gnomes -> Mecagnomes When Odyn killed Mimiron, it left many mechagnomes alone. Gelbin Mekkatorque found them during the Assault on Ulduar and decided to bring them back to Gnomeregan to study Mimiron's work on the Curse of Blood.
Dwarves -> Earthen: ?
Worgens -> Ulfarks: When Azothas settled in Tel'Abim, they spread in many tribes. Some of them where more Shamanic, and other more Druidic. One of these Druidic tribe explored the Emerald Dream, until they met the exiled Worgens and been cursed by their diseases. It took many years and sacrifices for Azothas to contain the curse, and more years to ease the contaminated Azothas, who called themselves "Ulfarks" after Vrykul's language.
Night Elves -> Nightbornes After the freedom of Suramar and the Legion's defeat, the Nightwell was slowly dying, leaving most of the surrounding city empty. Most Nightbornes tried to join Shal'Aran to use the Arcandor, but it created some tensions between Suramar refugees. Tyrande asked First Arcanist Thalyssra to join her in Darnassus, but only if she kept the healed Nightbornes. Seeing this as an opportunity to start things over, she accepted the offer and moved to the Night Elft capital, leaving the wretched at Shal'Aran.
Draeneis -> Brokens: Since Velen gave the Draenei's leadership to Nobundo, the foreseer reintroduced the Brokens into the Exodar.
Both:
Pandarens -> Tuskarrs: ?
New Feature: AltRace Class Combinations
Hord:
Gilblins : Warrior / Hunter / Rogue / Shaman / Druid
Yaungols: Warrior / Hunter / Shaman / Mage / Deathknight
Mok'nathal: Warrior / Hunter / Rogue / Priest / Shaman
Wretched: Warrior / Hunter / Rogue / Demon Hunter
Reincarnated: Warrior / Paladin / Hunter / Rogue / Priest
Zandalar : Warrior / Hunter / Rogue / Priest / Mage
Alliance:
Azothas: Warrior / Hunter / Priest / Shaman / Druid
Mecagnomes: Warrior / Hunter / Shaman / Druid (using mecamorph)
Earthen: ?
Ulfarks : Warrior / Hunter / Shaman / Monk / Druid
Nightbornes: Warrior / Rogue / Hunter / Mage / Warlock
Brokens: Warrior / Hunter / Rogues / Priest / Warlock
Both:
Truskarrs: Warrior / Hunter / Shaman / Druid / Death Knight
Starting Experience:
From Hord perspective: Zandalari start their journey by sailing to the West and find Kezan, where strange and mutated Goblins were living. These folks are assaulted by Nagas' raids, so they team up to join Orgrimmar.
From Alliance perspective: Tel'Abim is a long forgotten island, few are the sailors that walked on its shores, but the place has been discovered millenia ago by a young race : Azothas.T hey lived and prospered on Tel'Abim for many centuries, different clans were formed, but peace was maintained (even after the incident that touched many druids, changing them into savage beasts called "Ulfarks"). But a new threat appeared, and the peacefull island is assaulted by Nagas. Many Azothas took the sea and sailed to the nearby Islands to ask for help, and some of them found a human Nation called Kul'Tiras, that was assaulted by mutated Pirates and Nagas. Kul'Tiras' military fleet accepted to journey to Stormwind and ask the Alliance to help the Lost Islands' population.