The following was leaked to 4Chan, Reddit and MMO Champion's forum. Sounds legit.
Zul and Azshara are the main villains.
After the last defeat Zul has asked Rastakhan for more support, but Rastakhan has lost the confidence in Zul and refuses support. In a big coup, Zul has Rastakhan assassinated and assumes control of the Zandalari.
The Zandalari trolls ally with the naga, whose goal it is to flood the continents and raise the ocean level.
The big focus of the expansion will be on night elves and trolls, with Malfurion and Vol'jin being the main heroes.
This is Vo'jin's big opportunity to prove himself as the warchief of the Horde.
A big emphasis will be on Vol'jin's loyalty to the Horde, despite the opportunity to be part of a newly risen troll empire. His story will be about resisting the temptation to join the Zandalari and stay with his family.
This is the South Seas expansion that so many players want.
New class will be Shadowstalker.
Trolls, Blood Elves, Orcs, Goblins, Forsaken, Humans, Dwarves, Worgen, Night Elves and Gnomes will be able to be this class.
The class will be a ranged class that uses bows, crossbows and guns, and it will have a heavier focus on magic attacks.
Specs are Dark Ranger, Spirit Mending and Shadow Hunter. Dark Ranger and Shadow Hunter are dps specs and Spirit Mending is a healing spec.
New Shadowstalkers will start at level 1, but players who buy The Dark Prophet will be able to create 1 Shadowstalker character at level 100.
New levelcap will be 110.
The expansion will have 8 zones.
The Valarian Depths, Zandalar, Kul Tiras, Kezan, Tel'Abim, Plunder Isle, The Rift and Broken Isles.
The two starting zones will be The Valarian Depths and Zandalar.
The Valarian Depths are an underwater zone with naga, similar to Vashj'ir, but smaller. It serves as the intro to the storyline revolving around the naga. The story in this zone deals with Ozumat and the kidnapped Neptulon.
Zandalar used to be the capital of the Zandalari, before it was destroyed in the Cataclysm. This zone is completely ruined. Players will get here and figure out what happened to the trolls, and what they are up to.
After the starting zone players in each zone will board a ship that will lead them to either Kul Tiras for the Alliance or Kezan for the Horde. These zones will serve as the capitals for the factions, Players will get their own ship here, which they will use to travel around the islands. Around the capitals there will also be some questing.
Tel'Abim is a tropical paradise. A group of peaceful trolls, the Sunspear tribe, live here. The zone is mostly light-hearted. Players have to help the Sunspear tribe against a new pygmy-like race that is giving them trouble.
Plunder Isle is a pirate island. Here people can recruit pirates for their crew.
The Rift will be the first of two endgame zones. Here lies the naga city of Nazjatar.
The Broken Isles are a group of islands, and it is the second endgame zone, that is overrun by a mix of troll and naga forces. Here the naga and trolls are going through the ruins of the Tomb of Sargeras in search for powerful artifacts. The Tomb of Sargeras will be a dungeon.
Players will not only be able to travel with their ship from island to island, but also be able to explore the open seas and hidden islands. They will be able to recruit a crew of pirates. Depending on what pirates you pick, they will contribute in different ways to your ship. For example you can get a cook for your ship, which will unlock exclusive cooking recipes and who will be able to cook for you. Other pirates might contribute with different abilities, like professions or as a barber on your ship, or by offering rare quests.
Blizzard returns to creating multiple dungeons per zone. Expect about 2-3 dungeons in most zones.