Illidan Stormrage is coming back - that's the first detail we really discovered about Legion.
Illidan was, of course, one of the breakout characters of WCIII, probably comparable in popularity with Arthas. So when Burning Crusade was announced, it was pretty exciting to get to interact with him (well, for people who knew about him. I had only been playing WoW for a few months when BC came out, so I really didn't know who he was until later.) However, despite facing many of his forces throughout BC (though the allegiances were a little muddled - sometimes the line between Illidari and Legion proper were a little confused,) we really only saw him at the end of the Black Temple raid, or more accurately, the 1-2% of people who could actually make it that far through the raids saw him there. I believe he had an appearance at the end of the Netherwing reputation grind, but I've never gotten that far.
Anyway, for such a popular character, he got pretty short shrift.
We did manage to get another appearance by him in the Caverns of Time in Cataclysm, and we got a little vision of him in Felwood (fingers crossed that Feronas Sindweller will be a Demon Hunter champion, especially as he dates back to WCIII as one of the random names you'd get in multiplayer when summoning a DH.) But ultimately, we haven't really gotten a chance to do much with him. He was painted into a villainous corner, and while we've seen some of his more heroic acts, it's been very scattered. He died as a major boss in BC (though not the final boss, which was actually a bit odd) and hasn't done a whole lot since. So how is he coming back?
Here's what we know happened:
After we killed Illidan, Maiev took his body. As a half-demon (or arguably just a full-on demon, but one that was not bound to the Burning Legion,) Illidan's soul resides within the Twisting Nether, or possibly it would naturally return there. Physical bodies for demons are like really expensive clothes. It's a pain to get a new one, but they're ultimately replaceable.
Maiev understood this, and was not about to allow Illidan to just float off to the Nether and get a new body at some later point. So she used some sort of magic developed by the Wardens (probably to bind the Legion's demons) to bind his soul and body inside a great magical crystal and return it to the Wardens' prison on the Broken Isles (which will be a dungeon.)
As we saw in the cinematic teaser, Gul'dan B (from the alternate Draenor) finds this crystal and somehow activates Illidan.
To what end?
Illidan is pretty powerful. He was imbued with his powers by Sargeras himself, which suggests that there could be a ton of untapped potential there. There's also the fact that Illidan's soul and body might both be bound, but they might not be bound together. His soul is certainly selfish and doesn't really think through things too far in advance, but ultimately isn't evil in the omnicidal way one often sees in Warcraft. Illidan wants to be a badass, he wants to impress Tyrande (which, dude, that ship sailed like 10,000 years ago,) and I think he'd love to be respected as some world-saving hero - a bit of credit that his brother Malfurion never gave to him.
But his body in the hands of the Legion could be an incredibly powerful weapon.
The thing is, we actually don't know how Sargeras is doing these days. After Medivh was killed while possessed by the Dark Titan, we haven't heard of him doing anything since. Yes, it's a common bit of speculation that the "Dark Voice" commanding Varimathras in the Battle of Undercity was Sargeras, but they never said that explicitly.
Perhaps Sargeras has been missing. And perhaps that means that the Legion is not at full strength. They need some piece of their master's power. And perhaps that power rests within Illidan Stormrage.