I think the most encouraging thing I've heard out of Blizzard about Legion is that they are no longer shooting for an expansion every year. They tried that for basically a decade and it did not work - only leaving us with very long content gaps as we waited for the next expansion.
Knowing, then, that they are expecting Legion to last two years (or possibly more, but I'm going to assume it'll be two years,) what might that mean for the content added to the expansion?
We know we're getting the Karazhan dungeon, which will by Mythic-only, but thankfully Mythic dungeons are actually not that difficult to get into.
In addition to the Trial of Valor raid, which will presumably resolve the story of Stormheim and see Helya killed (it'd be insane if she escaped a second time,) we're also going to be getting more Suramar story content in 7.1 as a lead-up to the Nighthold raid.
Once the Nighthold is finished, though, where do we go from there?
Clearly, there's the Broken Shore and the Tomb of Sargeras. While we haven't done much there (though I realize that it can be explored right now - I just haven't been there yet,) this area is central to the Legion invasion.
The question then, is whether closing the portal will be the ultimate goal of the expansion or if that will be an intermediary step.
Additionally, we know that Illidan is going to be resurrected in some way involving Light's Heart. What his ultimate purpose in all of this is remains to be seen - we seem convinced that he's needed to defeat the Legion while Gul'dan claims that he is what will grant the Legion its ultimate victory. My tin-foil hat theory is that both plans involve Illidan serving as a vessel for Sargeras - the Legion wants to use him as a new, more powerful avatar, while the Army of the Light's idea is that if Sargeras could be trapped within Illidan and Illidan were somehow banished or killed, it would remove Sargeras from the board as well.
The stories on the Broken Isles will mostly be resolved after the three raids. The Underking was dealt with in a dungeon, the Emerald Nightmare, or at least Xavius, will be defeated in the Nightmare raid, Suramar will presumably be liberated with the defeat of Elisande and Gul'dan. Helya will no longer be a threat after the Trial of Valor.
We did thwart Azshara's plans to use the Tidestone of Golganneth to create the Wrath of Azshara (note that it was at about 10% health when we engaged it in the dungeon,) though she is obviously still around and still a threat. It is not unthinkable that we go off to the fight the Naga, but I'm still mostly convinced that Azshara will either headline her own expansion or be a major villain in a N'zoth-themed one.
If the Tomb of Sargeras is not the final raid of the expansion, it's possible we could head off to another world - while I think Argus could be its own expansion, we do have strong hints that we'll be going there eventually. After the Exodar quests after finding Light's Heart, if you start using the Dalaran teleporter but then interrupt yourself, you'll see Velen's Chosen (apparently an elite guard of paladins who already have their tier 19 sets) arrive and Velen calls for Romuul (who's apparently alive in this timeline) to finish repairing the Exodar so that they can finally go home.
If Argus is not a future expansion in and of itself (and oh man, if it is they'd have to make Mac'aree even more amazing and impressive than Suramar) I could imagine us going there and having our final raid against the very heart of the Burning Legion.
Obviously, with things like Karazhan, we could be seeing other, older parts of the world given new content. The invasion pre-launch event did a good job of making this fight feel global, but it might be good to keep emphasizing that.