There's huge upheaval after the battle at the Broken Shore within both factions. The event has been out for a week now, and we've had a full weekend. At some point we're going to have to declare all this stuff no longer spoilers, but simply the state of the game. But in case you didn't get to play this past weekend or week, I'm going to give it one more spoiler cut just in case.
Varian and Vol'jin are both dead. The Battle of the Broken Shore was a disaster - a demonstration of the way we underestimated the Burning Legion. We've fought them for a long time - some of us since the Third War, some since the War of the Ancients, and some all the way back to the Conquest of Argus.
But we didn't really get it. Every victory we've achieved in the past, every demon we've slaughtered - it didn't count. The demons aren't really there to die - their souls reside in the Twisting Nether, and getting a new body is no problem. Meaning we've been facing an unrelenting army that hasn't really suffered any losses.
We need to close the portal at the Tomb of Sargeras, but that's at best a delaying tactic. If we are to truly be safe from the Burning Legion, we need to find some way to strike at them in the Twisting Nether, to truly slay them where they live. We need to, as the Demon Hunter NPCs say, trust in Lord Illidan's plan.
Varian died a true hero - as his people were being slaughtered left and right and a massive Fel Reaver threatened to prevent the survivors of the battle from escaping, he dove off the airship to slay it, knowing fully that it meant his death. Taking out the Fel Reaver and several demons afterward, Varian was mortally wounded and then burned from within by Gul'dan's fel magic.
Vol'jin's death came slower, but it gave him an opportunity to set the Horde moving forward without him. Though he (understandably) does not trust Sylvanas, he claims that the Loa Spirits are whispering her name to him, and logically, he knows that if there's any leader within the Horde who has the experience to mount a desperate and difficult military campaign, it's the former Ranger-General of Quel'thalas.
As Vol'jin's body burns on his funeral pyre, Sylvanas asks who among the Horde wishes to help her avenge him. For the Horde, indeed.
Now, on the surface there are some great causes for concern. Sylvanas is, after all, the faction leader who could most arguably be considered a villain (ok, Gallywix too, though on a much smaller scale. Never really understood why Thrall allowed him to remain Trade Prince, but I guess it's water under the bridge.)
Sylvanas, after all, has used very Scourge-like tactics in her fights through Lordaeron. When asked (by an uncharacteristically contemplative Garrosh) what the difference between her and the Lich King was, she replied that she works for the Horde. That's not super reassuring. She also allowed Varimathras to build a secret faction loyal to him within her own city (and given that Varimathras is certainly still around, I really, really, really, no seriously really, want to see a confrontation between the two of them in Legion.) Her loyalties to the Horde have been tenuous at best, and with Garrosh gone, she has been the frontrunner for "faction leader most likely to go rogue."
There's also the fact that Vol'jin's listening to whispering spirits at a time when Orgrimmar literally has disguised Dreadlords trying to demoralize or convert Horde citizens. I get that as a Shadow Hunter (which I take to be a sort of hybrid Shaman/Hunter) you're constantly listening to the Spirits, but maybe this time you should really examine what they're suggesting you do.
But with all that said, this might actually be a good idea. It might be good for Sylvanas and good for the Horde.
Sylvanas has done some horrible things in the past, but her motivations - if she is to be believed - have been a fierce devotion to her people. She employs the Val'kyr to raise the dead and offer them a place among the Forsaken because without doing so, her people would die out. Forsaken can't exactly reproduce the regular way, and Sylvanas does not want to see the kingdom she built be overrun by the living, who of course have not historically been too forgiving of the undead - many of whom are actually just ordinary people trying to take back some semblance of their old lives.
The danger Sylvanas has posed to the Horde has always been that her loyalties to the Forsaken would outweigh her loyalty to the Horde. They are her responsibility, and she cares more about that than some group of big hairy monster-people across the ocean.
Yet by making her Warchief, that sense of responsibility is now extended to all of the Horde. "Her people" now refers to every member of the Horde. Her responsibilities are to all of them. How could she break away and abandon the Horde when she is its Warchief?
Sylvanas is not likely to put the Alliance at ease. Indeed, Genn blames Sylvanas directly for the abandonment that led to Varian's death, not to mention the much more valid reasons to hate her, like how she invaded his kingdom and killed his son. Vol'jin became Warchief after coordinating with the Alliance and ousting the Warchief who most directly wished to war against them. Sylvanas does not have any of that diplomatic precedent, and really more accurately as negative marks in that category.
But there's no one in the Horde more suited to waging a massive war than Sylvanas Windrunner. Perhaps the Loa Spirits were right.