The upcoming expansion is simply called Legion, and that carries with it some serious implications. The title of WoW's first expansion, the Burning Crusade, actually referred to basically the same thing - one could make the argument that the Burning Crusade is an action while the Burning Legion is a group of demons and one Titan, but the Burning Crusade is what the Legion does.
Of course, the Burning Crusade split its focus considerably. For one thing, the headlining villain (though not the main figure on the box art - mirroring Vanilla's Dwarf/Night Elf and Orc/Tauren art, BC had a Draenei/Blood Elf box) is decidedly not a member of the Burning Legion.
Illidan's relationship with the Legion is something Blizzard has evolved and probably rewritten a few times. Illidan waged a war against the Lich King, for example, at Kil'jaeden's behest (a war he lost, which allowed Arthas to merge with Ner'zhul to become the second Lich King.) Still, Burning Crusade was largely about us fighting the Illidari - Illidan's organization of allied Blood Elves, Naga, Fel Orcs, and of course his far more elite and trusted Demon Hunter disciples - the very characters who we will be able to play starting on August 9th (I'm really eager to take my Demon Hunter through Black Temple and see if there's any special dialogue there - theoretically everything inside should be friendly.)
We did fight the Burning Legion ultimately in the Burning Crusade, especially as it became clear that Kael'thas and his faction had abandoned Illidan in favor of the Legion (a move many were upset about, given that Kael'thas was pretty much a good guy in Warcraft III, his alliance with Illidan a pretty understandable move to survive.) Our ultimate showdown with the Legion was at Sunwell Plateau. This fight ended favorably, but there was absolutely no sense that there was any finality to it - Kil'jaeden was explicitly not dead. We had merely slammed the door in his face. (Meanwhile, we've killed Archimonde twice and both times it's been kind of unclear whether he's actually dead or not. My money is on no, even if Mythic raiders killed him in the Twisting Nether, which is supposed to mean perma-death for demons.)
With an expansion called Legion, where it has been made explicitly clear that we're facing down the Burning Legion's biggest invasion of Azeroth ever, though, I think Blizzard needs to actually make our eventual victory (and I'm assuming it's a victory, because there's going to be a final boss for us to beat) count for something. We can't simply "close the portal" again, because ultimately how would that differ from what we did in the end of Burning Crusade.
What might we actually accomplish by the end of this expansion?
We already know that we'll have killed Gul'dan B - he's the boss of the first major raid. The Emerald Nightmare is also probably going to be considered "dealt with," because even though Xavius isn't the root cause of the Nightmare, I really can't imagine going back in after a raid literally called the Emerald Nightmare. (I'm still really curious about the relationship between the Old Gods, the Nightmare, and the Legion.)
I think that bare minimum, we have to kill Kil'jaeden permanently. We know he's involved in this invasion and the whole "leaving him alive after BC" move was totally to set up a final confrontation with him like this. Archimonde might have a presence if he's still alive, though I kind of hope that when we killed him on Draenor, that counted to actually kill him permanently.
The big question is Sargeras.
Sargeras is literally the size of a planet, so I don't really see him being a raid boss in the conventional sense (unless we all become super-sized in some kind of cosmic arena.) Still, the Tomb of Sargeras, which I am 90% sure will be the expansion's final raid (not saying that the end of the raid will still be on Azeroth, mind you) is the resting place of Sargeras' avatar, which, while huge, is more in keeping with raid bosses we've seen in the past. I could see us fighting Sargeras within an avatar like this.
The question then is whether Blizzard is willing to pull the trigger on actually killing Sargeras off.
Sargeras has been the big bad of Warcraft since the beginning. It's only within very recent periods that we've discovered there might be anything truly more powerful and villainous than he is. The shapeless Void Lords are something we'll probably never even see, because of how final their position within the canon is (plus there was that comic suggesting Anduin's going to fight them in like seventy years, at which point I will literally be one hundred years old.)
However, eliminating Sargeras might not be the total collapse of the Warcraft story that some might fear.
First off, it doesn't necessarily mean the end of Demons as a threat. Demons existed before Sargeras created the Legion, and without their master, they might start doing their own thing.
Then you have the Old Gods - we've fought two of them in person, but it seems pretty evident that neither C'thun nor Yogg-Saron are actually dead. Y'Shaarj is (and I believe we cut off the source of the Sha with some unwitting help from Garrosh, even if that doesn't mean that the remaining Sha essence in Pandaria is totally gone.) N'Zoth we haven't even seen (except as a Hearthstone card.)
The Old Gods could absolutely headline an expansion - you could easily have N'zoth do one on his own (the other two surviving OGs I think would need to be presented as part of a package, as we've already faced them.) The Dark Below as a name isn't going to happen, but I think an underground expansion could be really awesome.
I keep wondering about Azshara - it'd be a pretty huge step down after Legion to merely deal with the Queen of the Naga. One could imagine her as a middle-tier boss in a N'zoth expansion, but on the other hand, it might be necessary to lower the stakes significantly after Legion so that things don't go totally insane.
And of course, there's always the possibility of new villains popping up. Garrosh was never mentioned before Burning Crusade, but his campaign to "make the Horde great again" actually made him one of the most nuanced villains we've seen in the game (even if it was infuriating to see the Horde falling into barbarism when you were playing as one of its heroes.)
I'm always going to hold out some hope that the Infinite Dragonflight gets a place in the spotlight, but I also know that that's more of a pet issue and not necessarily on Blizzard's priority list.
The other big question I have about the end of Legion is the fate of Illidan.
To my knowledge, Illidan is going to have to wait a while to actually be resurrected. I think that Gul'dan has his body, so perhaps we'll recover it following the Nighthold Raid, but then what?
Illidan is apparently going to be instrumental in the Naaru's plan to defeat the Legion (which now makes me wonder why they were organizing us to fight him,) and so I could totally see him playing a Thrall/Tirion Fordring role in the final boss fight, but then what?
Does he survive to lead the Illidari once again? Does he reconcile with Malfurion? Does he die to save us all? Can he even die, now that he's demonic enough to be bound to the twisting nether?
We'll have to see how the story develops as the expansion goes on.