So I realize that I've written several posts about future expansions, and ultimately Blizzard has managed, in the past, to pull the rug out from under me (while I predicted a Deathwing-centric one, I definitely did not predict Mists or Warlords, and Legion was one that I always knew they'd eventually have to do, but always put that in the "eventually" category.)
With Legion, there is a real question of where we go from here. After Wrath of the Lich King, a lot of players were talking about how it seemed like there was nowhere to go after Arthas, but those of us who knew a bit more about the lore knew that there would have to be figures like Deathwing, Cho'gall, Xavius, Azshara, and of course Sargeras for us to face.
Three of those are checked off already (Cho'gall twice!) And Sargeras is probably going to be the final boss of Legion - in one form or another (my money's on "while trapped in an Illidan Stormrage-shaped prison.")
Given that Sargeras is the Big Bad of the Warcraft universe, those questions that people were asking about a post-Arthas world are starting to feel like they've got more traction.
Now, all this being said, the Warcraft universe is a diverse one - unlike Diablo, where everything ultimately boils down to either angels or demons, Warcraft is a lot more like the real world in that there are unrelated cultures and threats that don't depend on each other to exist. Beating the Scourge did nothing to harm the Legion, Old Gods, Deathwing, or any other major threat. And when we got to Pandaria, it turned out there were people like the Mogu to worry about that we had never heard of before.
And on top of that, just because a leader dies does not mean that the whole organization is gone. That's something we've learned pretty painfully in the real world time and time again. We had to install Bolvar Fordragon as the new Lich King lest the Scourge just go rampant, but there are strong hints that while he might not be trying to take over the world right now, this Lich King is not exactly the same lawful good paladin he used to be. Demons existed long before the Burning Legion came to be, and without Sargeras to threaten them with annihilation should they stray from his leadership, demons could be a whole new thing - we might not encounter the kind of concerted effort to invade and conquer, but we'll probably be dealing with a million different factions of unpredictable demonic gangs - and let us not forget that the Dreadlords might go back to serving the Old Gods...
So now we move into prediction time:
Rather than speculate on the myriad possibilities for expansions, I'm going to make my guess at the next one. It's no guarantee, but Blizzard has really laid out some serious groundwork.
For an expansion about the Burning Legion, Legion is filled with references to the Old Gods. I won't go into every bit, but I seriously think that we are being manipulated into using the Pillars of Creation. While these may indeed do their stated goal - closing the portal to Argus - I am about 99% convinced that they are also going to serve as the keys to the locks on the prisons of the Old Gods. C'thun and Yogg-Saron might be dead (or they might be regenerating after we beat them) but N'Zoth is, as far as we know, in perfect health. N'Zoth also just awoke from the Emerald Nightmare, meaning that he's likely to start taking bolder steps in the physical world.
We have also seen a larger presence of Naga in Legion than we have recently. The Naga are of course remnants of Azshara's inner circle, and Azshara herself appears in Azsuna. The Naga are ostensibly helping the Legion - it's less explicit in Azsuna, but we know some will be fighting for them in the Tomb of Sargeras.
And on the surface, this makes sense. Azshara was on team green in the War of the Ancients, and perhaps she never abandoned her plans to marry Sargeras (however impractical that might be.)
But that was ten thousand years ago. And all that time, the Naga have been, well, Naga. Transformed by the Old Gods after the Sundering flooded Ancient Kalimdor, the Naga have been allied with the Old Gods for far longer than they were actively engaged with the Legion. We know that they fight alongside each other thanks to the events in Vashj'ir.
N'Zoth in particular is the Old God associated with the deep sea, and thus is probably the one who played the biggest role in the transformation of the Naga. He's also the Old God who stands to gain the most (because he's unambiguously still alive) by manipulating us into freeing him (using the Legion to provoke us into doing so.)
Ok, so we've got a main villain (N'Zoth) and a major raid boss (Azshara - who could be the final boss of the first or second raid tier.) We have a setting (the South Seas - islands surrounding Ny'alotha, which is presumably where N'Zoth is imprisoned.)
And we potentially have a new playable race in the Naga. I know there are lots of mechanical issues to work through with Naga (like pants and shoes and mounts) but lore-wise, Blizzard has explicitly said that while most Naga are loyal to their queen, they are still individuals with their own ideologies and factions. Thus it is not impossible that a group of Naga would break away from Azshara and potentially join the player factions. I don't know if they'd make it a Pandaren-style neutral race or if they'd come up with a different race for the other faction to play, but playable Naga are something Blizzard has talked about.
We're still many months from Blizzcon, but I really think we could see this or an expansion like it as the next one after Legion.