I'd like to think that Next expansion will see the return of the Black Empire with the rise of Nyalo'tha, N'Zoth's city where he's been imprisonned by Titans.
The shutting of Legion's portal (or destruction of Argus) will cause a huge shock on Azeroth and free N'Zoth from the depth of the South seas (starting south of Thal'Dranath to North Pandaria and East of Eastern kingdoms). He'll be in his fortress in the heart of Nyalo'tha and will send his minions, helped by Azshara and the Nagas, to invade to surrounding islands.
This will be a sea themed expansion.
Alliance :
Jaina Proudmoore visited KulTiras and they discovered an island populated with the Azotha, the first breed of cursed Vrykuls.
Magni Bronzebeard is awaken and offers Moira Thaurissan and her Dark Iron clan a place in Ironforge since their home has been invaded by Orcs.
Mimiron found a remedy for the Curse of Flesh and offers to help the Leper gnomes, changing them into cyborgs.
Velen named Farseer Nobundo for his replacement and his first act is to take the Broken in Exodar.
Tyrande granted the rebel Nightbornes to join her in Darnassus.
Genn Greymane cured the Emerald Nightmare's worgen with the same magic he used on the Nightbane Pack from Duskwoods, and offers them a place among the Gilnean pack.
Hord :
We'll discover that Vol'Jin was helping the Zandalari (they found an new island to settle but full of Nagas), he offers them a place in the Hord.
Baine offered the Highmountain taurens to join his side.
Sylvannas found a way to rise fallen high elves of Silvermoon with the help of Helya.
Since the end of Warlord of Draenor's events, Draeneis and Orcs made peace. Saurfang offered a place to the first borns of this mixed race.
Feeling that the oceans would be a huge source of treasures, Trade Prince Gallywix offered a place to the Gilgoblins, in order to make them collect underseas resources.
Lorthremar, asked by Sylvannas, offered a place to the High elves that Jaina Proudmoore dismissed.