Well, now that Nighthold is out, I think we start talking about the spoilers for the end. If you didn't see it, a video of the cinematic that runs after beating Gul'dan was actually unearthed well before the release of the raid. Now that it's out, I think we can talk about it with only minimal spoiler warnings.
So spoiler alert.
Legion has been out for about five months at this point, and one character who has been notably absent is Illidan Stormrage. Now obviously, he's been dead for a while, at our hands in Burning Crusade. But as we discovered in Legion, his body has been held in a crystal created by Maiev Shadowsong which was kept in the Vault of the Wardens along with the rest of the Demon Hunters. While Maiev was able to release the Illidari to aid in the fight against the Legion, she was unable to stop Gul'dan and Cordana from stealing Illidan's body.
We also know that Illidan's soul has been preserved, and in fact Demon Hunters get to communicate with him briefly during the class campaign.
At the top of the Nigthold, Gul'dan is attempting to weave a complex spell that would use Illidan's body as a vessel for Sargeras to come to Azeroth. Presumably, this would turn Illidan's corpse into a kind of Avatar. Instead, however, we defeat him, and as he lies broken and beaten, the rift into the Twisting Nether collapses. Not only that, but the crystal holding Illidan shatters and his own soul is able to re-enter it (I assume.) Illidan burns Gul'dan away much as Gul'dan had done to Varian and crushes his skull (no powerful artifact this time!)
With Illidan returned and ready to fight on the front lines against the Legion, we find ourselves at a very new chapter in our war.
Obviously, we know the next two major steps. We'll be hitting the Broken Shore to plug up the gateway at the Tomb of Sargeras. But we're also going to head to Argus after that.
With Archimonde defeated in Warlords of Draenor (though, infuriatingly inconclusively) and with Kil'jaeden as the final boss of Tomb of Sargeras, that really seems to only leave one possibility for the expansion's final boss.
But how the hell are we supposed to fight Sargeras, who, as a Titan, is literally the size of (or larger than) a planet?
Well, let's talk X'era's quests for a bit. The Naaru X'era sends us to take a look at Illidan's past and learn about his motivations and his flaws. She's a bit of a cheerleader for him, but if you really look at it, much of his life has been about failing to learn a lesson, namely what it is to sacrifice for the greater good. The Black Rook Hold scenario is particularly telling. Illidan sacrifices his forces in order to empower himself. And while yes, it does allow him to win in the end, it's ultimately pretty selfish. He effectively murdered his own forces. At the very most, he made tough calls in order to get the job done, but he did so in such a way that he came out as the victorious survivor, rather than anyone else. Regardless of its effect, this is not really sacrifice on Illidan's part.
In the Cataclysm-era Badlands, there is a quest chain in which Loramus Thalipedes is restored to life and then uses his body as a vessel to trap a demon named Razelikh within himself. The player than kills both of them, sealing them away in the Twisting Nether. As it turns out, when the Legion invades Azeroth they grant a new body to Razelikh, but because Loramus and Razelikh are now two souls attached to one another, Loramus finds himself in this new body with the demon and wrests control (mostly.)
But the point is, there's a precedent for a demon hunter taking another being into themselves in order to serve as something of a prison.
In order to enter Azeroth, Sargeras seems to need to put his essence into an avatar - otherwise he'd just be a floating Titan in space - which would be fine if he really just wanted to destroy Azeroth, but the lore surrounding the Scepter of Sargeras suggests his intentions are more complicated than that. Gul'dan intended to use Illidan's body as the vessel for Sargeras. But while that would be disastrous on Azeroth, perhaps it's something we'll have to do on Argus.
To beat the Legion, we need to defeat Sargeras. And as long as he's an ethereal space god, that's probably not possible. For Illidan's story arc to resolve, he's going to need to learn the meaning of sacrifice.
Thus, if we want to beat the Legion, I believe it will require Illidan to lay down his life and take Sargeras into himself. And then it will be our job to kill him.
So I think the final boss of Legion is going to be sort of Illidan and sort of Sargeras. We will fight Sargeras in the form of Illidan Stormrage. And in death, Illidan will have served the grand purpose he was always meant to - to seal away the Dark Titan once and for all, though his life and perhaps even his soul will be forfeit.