Many players of World of Warcraft even in its early days were mainly familiar with the events of Warcraft III - Illidan and Arthas were far more recognizable characters than say, Blackhand or even Gul'dan for these people. When Wrath of the Lich King came out, some asked where the game could possibly go from there. Of course, those who either had played more or simply delved deeper into the lore (myself being the latter - WoW is the first Warcraft game I ever played) could rattle off a few - Deathwing, for one (and of course he came immediately after the Lich King.) But the obvious "ultimate bad guys" were the Burning Legion.
The expansion coming out tomorrow (two days from now as I write this) is simply called "Legion." Unlike the Burning Crusade, whose title did imply an ultimate confrontation with the Legion but in reality was split between the Legion as a greater outside threat and Illidan's forces as the true headliners, Legion the expansion really does seem to be putting the Burning Legion front and center.
In Wrath and Cataclysm, the survival of the world was truly at stake (actually, you could argue the end of BC in Sunwell Plateau it was as dire as it is now - though I'd assume that a failure in SWP would have meant what succeeded already at the Tomb of Sargeras - that that raid was our successful nipping the invasion in the bud.) But after Wrath threatened to turn everyone in Azeroth into the undead and Cataclysm threatened to turn the surface of the world into a burned-out husk, we needed to de-escalate lest the high stakes get away from us entirely.
So in Mists of Pandaria, the focus was turned around on us - our conflict, Alliance/Horde, is what caused the resurgence of the Sha in Pandaria (though I'd argue we just revealed a problem that was already there,) but the threat was not of some global extinction, but rather the conquest of a brutal dictator. Had we failed to stop Garrosh, some small elite (mostly Orcs) might have actually prospered - though those people would be complicit in what is actually a textbook example of fascism. Still, life would have continued on Azeroth, and Garrosh's tyranny might have ended in some other manner later on.
The threat in Warlords of Draenor, at least to begin with, was similar - rather than Garrosh as our dictator, Grommash would have been. But the Iron Horde only briefly made it into Azeroth. Much of our efforts after the first moments of the expansion were to ensure the Iron Horde could not ever recover from its defeat in Tanaan and to protect the innocent people of Draenor as long as we were there.
That said, our success against the Iron Horde really brought about the larger problem we have now - in literally the third quest of the expansion, we free Gul'dan, and that single action is why we're now facing the biggest threat that we ever have in World of Warcraft.
If the "stakes dial" was turned down a few notches in the past two expansions, it has now been turned all the way up.
And at this point, the people asking "where do we go from here?" have a pretty reasonable time to start piping up again.
The first question to ask is what will happen as the expansion develops. At this point we know that we'll be defeating Xavius and Gul'dan in their respective raids. But while Xavius has a great deal of control over the Nightmare, he is not its original source. We also don't know if Gul'dan will die in the Nighthold (though after what he did to Varian, I'd really like to make that happen.)
From there, though, there's the question of just how far up the Legion leadership chain we climb. We don't even know if Sargeras is truly running things at this point - last we checked, he's been missing since Medivh's head was cut off. That being said, it was probably his voice we heard in the Battle of Undercity quest (really wish they could bring that back as a solo scenario,) so perhaps he is still calling the shots.
Could we kill Sargeras? Right now, it looks like most of the quests are focused on closing the portal at the Tomb of Sargeras - which I'm reasonably confident will be the final raid of the expansion. Given that the Tomb is where Sargeras' Avatar was buried, it might be that we fight that as the final boss instead of the planet-sized Titan himself.
This would mean that we had stopped the invasion, but it would still leave the Legion out there. Granted, that would allow for a future expansion that takes us to Argus (the Draenei/Eredar homeland,) which is something that players have speculated about for a long time, but it would mean that our victory in this expansion would be at best another holding action.
Another question to ask is what exactly Gul'dan wants to do with Illidan's body. In the Demon Hunter starting experience, Gul'dan claims that Illidan is the key to the Legion's victory - which is funny because the Illidari at least are convinced (and I believe according to the Illidan novel, the Naaru are as well) that he's the key to destroying the Legion.
Illidan's role in the expansion is anyone's guess. We do sort of want to see him redeemed, or at least given credit for not being a full-on villain like Arthas or Deathwing. One tinfoil hat theory that I just want to put down for the record is that Illidan could be the final boss of the expansion - not because he's gone evil, but because he has taken in the spirit of Sargeras, and he wants us to kill him to seal them both away (basically like the Loramus Thalipedes quest in Blasted Lands, but on a much larger scale.)
Yet another question is this: just how loyal are the demons to the Legion? Remember that the original Legion (before the Eredar and other races were transformed into demons) were all coerced into joining - Sargeras had discovered how to permanently kill demons, and threatened to do so to anyone who didn't join.
It appears there are some quests in Legion that actually suggest that there are elements within the Legion who would be willing or even eager to desert Sargeras. Some demons might even start looking to powers that are on the good side of the spectrum.
A post-Legion world would be pretty interesting - Blizzard would be free to invent all sorts of new factions of varying antagonism. Demons are beings of chaos, but perhaps that might mean that in the future we have mercenary-like demons who are willing to aid us (or pay us for aid) in their affairs. It would make a Warlock's life a lot more complicated, that's for sure!
In terms of "where we go from here," the obvious answer is the Old Gods. We have "killed" two of them, but only one is actually, truly dead (and that one died before pretty much any of the playable races existed.) C'thun and Yogg-Saron are clearly just regrowing and regrouping, and we haven't even laid a finger on N'zoth yet.
But as we learned in Chronicle, the Old Gods are themselves servants to a higher power - the Void Lords. We don't know exactly how these relate to, say, Void Gods formed from darkened Naaru, but we do know that the Void (and Shadow magic) are something clearly distinct from the chaotic Fel magic that the Burning Legion favors.
Post-Legion, an Old God-themed expansion seems like an obvious choice, though they might want to again lower the stakes somewhat so that there's time to reset the "stakes dial" after Legion turns it up to eleven.
It might be like a broken record at this point, but a South Seas expansion in which Azshara is the main villain is another one of those obvious things for Blizzard to do. A bit like Xavius, Azshara straddles the line between being associated with the Legion and the Old Gods, but while Xavius is clearly on team green, Azshara's loyalties (if they are to anyone other than herself) seem more to be on team purple.
The question is whether the South Seas expansion would be the one in which we face off with N'zoth, or if that would get its own. It is odd, actually, that neither Old God boss in WoW was its expansion's final boss. C'thun has sort of become the final boss of vanilla (yes, not an expansion, but you know what I mean) but only because the original Naxxramas was removed. Given how incredibly powerful these things are, it seems that N'zoth at least should be able to get his own expansion.
Personally, I think there's a lot of potential for an underground-set expansion. Everyone was a little disappointed when Azjol-Nerub was reduced from a whole zone to a pair of dungeons. But if you made an entire expansion of Underdark-style stuff, you could probably invent plenty of stuff to fill up the "continent" in which it was set.
So after Legion, I can pretty clearly imagine at least three expansions - South Seas with Azshara as the main villain, "Underground" with a theme of N'zoth and his minions (with some Scourge-affiliated Nerubians as well!) and Argus, with Kil'jaeden and possibly Archimonde (assuming he didn't perma-die in Mythic Hellfire Citadel.)
Granted, both Mists of Pandaria and Warlords of Draenor sort of came out of the blue. Blizzard obviously can just come up with new lore as they need to. If Legion winds up being as good as it looks like it will be, WoW can probably afford to have new expansions for years to come, so unless they do something insane and simply decide to end the game at some point, I think we can confidently expect to see ideas like these or something similar.
And of course, I want an Infinite Dragonflight expansion, but I'm not going to hold my breath.
So as this posts, it will be fourteen hours until Legion officially launches in the United States. I'm still currently trying to decide on whether to get the Ashbringer or Truthguard first, and the zones that I go to will, as I stated earlier, be determined by the roll of a die.