Here's a picture from MMO-Champion (the thumbnail should be small enough to prevent spoilers.
Spoilers to follow (nothing huge if you've been following the Alpha.)
Looks mostly familiar, but check out the names for the Alliance and Horde sides of the city. These are not the Silver Conclave and the Sunreavers. Instead, we have Windrunner's Sanctuary for the Horde and Greyfang Enclave for the Alliance.
Piecing things together from the Alpha, it looks like both the Alliance and Horde are losing their leadership. Vol'jin will merely go missing - I doubt he's dead, because he only just became Warchief. But with him MIA, Sylvanas becomes acting Warchief, which obviously does not bode well for Alliance/Horde relations.
The Alliance gets it worse (don't they always?) and it appears that Varian Wrynn will die in the Battle for the Broken Shore. How he dies remains unknown. There are some rumors that Sylvanas does it, but I hope these are false, because A: it's yet another instance of the Horde getting one over the Alliance, B: it's not like Sylvanas needs to do anything else to make the Alliance angry with her, and C: I'd find it far more interesting if Sylvanas had actually tried to be a reliable ally in the fight against the Legion (as she appears in the cinematic) only for everything to go wrong anyway. (In fact, the rumors that she kills Varian may be in-universe rumors as well.)
Anyway, Sylvanas as Acting Warchief explains Windrunner's Sanctuary.
But Greyfang Encalve?
The rule over Stormwind will certainly go to Anduin if Varian does indeed die, but that doesn't necessarily mean that he becomes High King of the Alliance. As the leader of Gilneas, Genn Greymane has a legitimate platform to seek High King status. Couple that with the fact that he's actually the only leader in the Alliance left who was part of the original Alliance of Lordaeron (Magni's still in stone form as far as we know, and was it his dad who was in charge at the time?) Sure, Genn quit that Alliance, but he'd say only temporarily.
The thing is, regardless of who is actually the most qualified to lead the Alliance (probably Tyrande,) Genn is going to take Sylvanas' rise to Warchief as a personal affront, and I suspect that he'll basically strongarm the rest of the Alliance into following him.
This is a pretty huge deal, and might really change the tone of Alliance questing in Legion. It's also probably one of the reasons that the Class Orders will have to pick up the slack, as I totally would not put it past Genn Greymane to obsess over his revenge even when there are demons pouring onto the surface of the planet.