For quite a while, I was convinced that the Tomb of Sargeras would be our final raid in Legion. I was concerned that there wasn't an obvious "middle tier," and that the design of the expansion was built, like Warlords, around only two raid tiers.
Now it's possible that this was the case in early designs - when Warlords came out, Legion had to have already been in at least early stages of development, and at the time Blizzard was still talking about yearly expansions that would, inevitably, have to be smaller. Of course, Blizzard demonstrated that even with a smaller expansion, it still takes about two years to get through the cycle. I don't think players were all that excited to pay 50 or 60 bucks twice as frequently either. But it's possible that the original concept for this expansion was to have us hit the Nighthold, then the Tomb of Sargeras, and then move on.
Thankfully, at Blizzcon they confirmed that that is not how things are going this time.
The middle tier is the Tomb of Sargeras, and we know that it will be a 9-boss raid (which is a bit on the small side, but remember that Black Temple also had 9 bosses, and that was a pretty popular one. We're not talking Trial of the Crusader here.)
We don't yet know a ton about the Tomb of Sargeras, but we do know that it will have Kil'jaeden as its final boss, as well as a reanimated Avatar of Sargeras (the reason it's called his Tomb even though he's definitely not dead) somewhere in there as well. We also know there will be at least one Naga boss, though not Queen Azshara (I suspect that she's either going to be the final boss of a raid or she's going to have her own expansion - it really depends on whether we fight N'Zoth in an aquatic setting or in an Underdark-style setting.)
But this leaves us with an interesting question: Who is going to be Legion's final boss? And there's one answer to that that seems as inevitable as it is impossible:
On the pro side:
1. Sargeras is the master of the Burning Legion. This expansion is called Legion, so it would be a bit of an anticlimax for us to face anyone other than its creator and master.
2. The next rung down on the Legion's hierarchy is occupied by two individuals - Archimonde and Kil'jaeden. Archimonde was the final boss of Warlords of Draenor, and we will have just beaten Kil'jaeden in Tomb of Sargeras. Both characters will have already been beaten in two raid fights each.
And on the con side:
1. He's by far the most powerful entity we've heard of (the Void Lords might beat him - might - but we haven't really had any real interactions with them.) This is a guy who killed multiple Old Gods with a single blow (plus the planet they were on and the Titanic World-Soul that rested within said planet.) Remember that the Pantheon didn't kill Azeroth's Old Gods - not because they couldn't, but because they were too powerful and risked damaging Azeroth itself. Now, being powerful doesn't disqualify someone, especially when they'd be the final boss of an expansion. But it's hard to imagine anything at all feeling like a threat after we've faced Sargeras himself. WoW isn't going to end until it's totally unprofitable (and if they keep putting out expansions like this one, that isn't going to happen for a long time.) Sargeras would be the obvious answer to who the ultimate final boss of WoW would be, but I don't see Blizzard ending the game until they have to.
2. He's literally the size of, or even bigger than, a planet. In fact, Sargeras is, kind of literally, a planet. Presumably, eons ago, there was a planet Sargeras that eventually hatched or transformed into the Titan who would later go insane. We don't know exactly how the process works (something we should find out about before Azeroth emerges) but the point is that physically, Sargeras is unbelievably huge. Like, the speed of light probably plays a significant role in how fast his brain can communicate with his limbs (though, as a being of arcane magic, he might be able to bypass that.) As a tank, I'm up for tanking anything, but where would I even stand?
Of course, my ongoing theory about what role Illidan is going to play in this expansion just seems to be getting more and more plausible or even necessary. To reiterate:
Illidan's story is all about how he doesn't really get sacrifice - he's been willing to take huge costly risks, but it's always someone else who pays the price for him. Loramus Thalipedes showed us in Blasted Lands that by drawing a demon into himself, both can be banished (though this banishment was not permanent, as Demon Hunters find out early in their class campaign.) So my theory has been that Illidan's sacrifice will take the following form - he'll draw Sargeras into himself, and then we will fight and kill Illidan, sending both the Betrayer and the Dark TItan into the abyss together.
So that's my narrowed list - we fight either Sargeras proper, or Illidan possessed by Sargeras. I really can't think of anyone else who would fit the bill.