Legion has been an enormous expansion in terms of stakes, content, and lore. The Legion has loomed over the World of Warcraft really since Warcraft 1, and the notion that the Legion could ever truly be defeated and ended seems hard to imagine, and yet here we stand in the best position the game will ever get to ending it.
What comes next?
Well, Blizzard has put a lot of effort into expanding the cosmos of Warcraft, and one aspect is the Old Gods and more importantly, the Void that they serve. While the status of Old God bosses C'Thun and Yogg-Saron is somewhat ambiguous (we killed them, but did we really kill them to a point where they can't come back? That which is dead may never die, and in strange aeons even death may die,) it's plausible to consider N'Zoth to be the last one standing (or flomphing or whatever a continent-sized-tentacle-monster does.)
Speculation purely based on lore existing even prior to... like Mists... has suggested that an expansion set in the South Seas that sees the return of Kul Tiras and a fight against Azshara and possibly N'Zoth would make perfect sense. We might be seeing more evidence for this based on information from the 7.3 PTR.
Spoilers to follow.
Dialogue involving Alleria and some of the Eredar resistance fighters (I think - they seem to be an independent subset of the Broken, who themselves were transformed by red mist strongly implied to be derived from the blood of Sethe, which cursed the Outcast Arrakoa on Draenor, and in turn Sethe seemed to be linked in some way to the Old Gods or the Void) seems to suggest strongly that while the demons and the Fel will obviously be a core aspect of the fight on Argus, there will be a strong presence for the Void - one that is not necessarily opposed to us.
Given the tendency since Mists to kind of preview the themes of the next expansion within the events of the latter patches of the contemporary one, this strongly implies that the expansion to follow deals largely with the Void (Shadow Priests should be happy about that.) Given that the Old Gods are creations of the Void, focusing on N'Zoth would make perfect sense. We even have a small Naga wing in the Tomb of Sargeras raid, which is almost a way of saying "hey, keep these people in mind... just in case they pop up again."
Now, in the 7.3 PTR there's also a tiny file that seems to be some textures that are labeled as having something to do with Kul Tiras. This might seem really minor, but the very first mention of the Iron Horde was at some point in the 5.4 era when there was an almost featureless grey texture labeled Iron Horde, which I think we all assumed was just something to do with the "True Horde" that was loyal to Garrosh in the Siege of Orgrimmar. (Pro tip: always be really wary of any organization calling themselves the "true" version of whatever original organization they're talking about.)
Gamescom is this summer, and of course Blizzcon is in the fall, so we're actually pretty close to being due for an expansion announcement. At this point I would find it surprising or even shocking if we don't see some kind of Naga/N'Zoth, nautical-based expansion with a heavy Old God theme in the South Seas following Legion.
Place ya bets!