WoW's earlier expansions really built themselves around the clear final boss. Burning Crusade, Wrath of the Lich King, and Cataclysm head a real headlining villain, and much of the expansion was built out from there.
Now, I'm sure you notice a problem here: Illidan Stormrage was not the final boss of Burning Crusade.
Blizzard has gone back and forth on just how much the Sunwell Plateau was built into the concept of BC from the start, and how much of it was created to deal with a massive content drought. You see, Burning Crusade launched with way more content than you'd typically expect from an expansion's X.0 release. It was the first expansion, of course, and I think at the time they expected that they'd try to get as much content out in the beginning and be able to use the time after launch to work on the next one.
The weird consequence here was that tiers 4 (Karazhan, Gruul's Lair and Magtheridon's Lair) and tier 5 (Serpentshrine Cavern, the Tempest Keep raid) both came out simultaneously, with the launch of the expansion. Black Temple and the Mount Hyjal raid both came out not too long afterward, in 2.1. Given that there was nothing like LFR and in fact raiding itself was kind of by definition a hardcore activity, there was basically no one actually "prepared" (get it?) for tier 6 when it came out, as they were, at best, still working on tier 5.
But after enough time, they did go on to do Black Temple, and Illidan fell. And then there was practically nothing for over a year.
Ok, that's not really true. 2.2 brought in-game voice chat (that no one ever used,) and is basically the antecedent to the non-patch of 6.1. But 2.3 brought Zul'Aman, a ten-player raid to give something to guilds that enjoyed Karazhan but had never been able to transition to 25-player raids. The thing is, while Zul'Aman was challenging and rewarding, it didn't offer much for the successful 25-player guilds who had been farming Black Temple for months.
So it stands to reason that Sunwell Plateau was built as a bonus tier to give people something while they waited for Wrath. Wrath would then go on to stagger the release of content much better and establish the "one tier at a time" pattern that WoW has stuck to since then.
Anyway, the point is: though Blizzard sometimes says that Sunwell was always going to be the final raid (and to be fair, the scene you get after turning in the item that drops off of Kael'thas in Tempest Keep does set up his return,) it seems more likely that Illidan was meant to be the final boss, and Kil'jaeden kind of came in to provide a suitable step up given that Illidan had already gone down.
I'd like to think that expansions are better planned-out these days, but Warlords kind of suggests otherwise. Really, my hope is that Warlords will be seen as a kind of fluke of poor planning, rather than a dismal sign of the game's future. Only time will tell!
But I do think that having a good sense of who the ultimate villain of Legion is would be a good route to go. There can be twists and surprises, but there needs to be some build-up. As sick as I was of Orcs by the end of Mists, they did manage to build up Garrosh as kind of the most extreme representative of the stupidity of the Alliance/Horde conflict. Archimonde, however, had almost no presence in Warlords - Gul'dan would have made a lot more sense as the final boss, but they appear to have been saving him to kick of Legion, and I guess they didn't want to do things exactly the way they had done with Garrosh.
So, after all this build-up, who do we think would work as a final boss for Legion?
His face is on the box, and it's pretty clear that Illidan is going to be one of, if not the most central figure of the expansion. Essentially, they're going to finally do right by Illidan, allowing him to have an actual arc after he spent practically all of Burning Crusade - which was theoretically his expansion - cooped up in Black Temple. Still, unlike Arthas (who has always felt like kind of his counterpart, I guess because both were breakout characters of WCIII,) Illidan was always more of an antihero than a pure villain. Yes, he consorts with demons, but he always fought against the Legion in favor of Azeroth. He's super reckless and certainly dangerous, but on the scale of Warcraft characters, is he really evil? Not really. If anything, I'd expect him to play more of the Tirion/Thrall and the Aspects/Khadgar, Grommash & Yrel role, with him aiding us in the defeat of the final boss. I'm sure he'll anger a lot of people and probably get some people killed, but I think this is going to be his redemption story (of course, I thought that might have happened for Garrosh, and we all know how that turned out.)
We know that Azshara will have some influence on the events of Legion, but to what extent she actually enters the fray, that is uncertain. She seems to be using a proxy to lead the Naga in Azsuna, which I think suggests that she'll still remain hidden in the shadows - perhaps having a quick appearance like she did in Darkshore or even casually exiting a fight like she did in the Well of Eternity dungeon, but Azshara could easily headline her own expansion, and I think Blizzard would be wise to hold her in reserve.
This is flat-out not happening, though Gul'dan will probably serve as the primary villain of the leveling experience. But he's going to be the first raid tier's final boss, so while it'll be a rather epic fight, he's not going to be there for the end of the expansion.
Ok, to be frank I hate all the theories that Khadgar is somehow evil. Just because he's got a wry wit and isn't a volcano of testosterone-fueled fury, people don't trust him. That said, just for fun, if Khadgar is somehow really Medivh in disguise, and Medivh is still possessed by Sargeras, then... why the hell would he have helped us during the Third War? Seriously, this rumor needs to die. Though he did release Gul'dan...
We just fought Archimonde, who has now "died" to us twice (assuming those tweets about there being only one Legion across all timelines is canon.) I also think that, much for the same reasons they didn't have Grommash serve as the final boss of Warlords, they're not going to have Archimonde do it twice in a row. But that also probably rules out Kil'jaeden, as it'd be a bit silly to have us fight two Eredar Lords one after another as final expansion bosses. This does set up a bit of a problem though, because this is the expansion named Legion, and we should be fighting the Legion's biggest bad guys...
Here we go. Sargeras is the big bad of essentially the entire Warcraft universe (though the Old Gods could rival him.) The Legion was his creation, and he's been trying to get to Azeroth for over ten thousand years. The only reason I'm hesitant to fully predict him as our big bad is that this could potentially be a shark-jumping moment for the series. Where do you go when Sargeras is killed? Certainly you could build up N'zoth as the new big bad, but you run the risk of losing the traditionally biggest threat to the universe. If only there was a way to have their cake and eat it too...
Avatar of Sargeras:
Ah ha! Sargeras has never been able to physically set foot on Azeroth (or if he ever did, it was before he went evil - assuming that Azeroth existed already when that happened.) However, the closest he came was when he sent an Avatar to Northrend. This was, I think, about a thousand years ago (Aegwynn was very old when she gave birth to Medivh - she preserved her youth with magic. The Tomb of Sargeras is actually not truly the Dark Titan's tomb, but is instead the resting place of the physical body he had created to scout of Azeroth and trick Aegwynn into letting him possess her and her son in turn.
Given how much of the expansion centers appears to center around the Tomb of Sargeras and the fact that the Tomb is not listed as a launch dungeon or raid, it seems like a prime candidate for the expansion's final raid. This would allow us to fight Sargeras and win without permanently removing him from the narrative.
So if you ask me, the Avatar of Sargeras seems like the perfect compromise, giving us the clear "most powerful villain we've fought to date," and letting us very definitively halt the Legion's invasion in its tracks, but letting us save the ultimate destruction of the Dark Titan and the Legion for when we go to Argus ten years from now (or whenever.)