I could make arguments for a few characters…
Anduin: He goes in and tries to talk her down, hoping for some form of redemption or to try to get her to put things right while Tyrande yells at him to just kill her. Turns into a moment of character development counter to Varian’s in WotlK with the message “not everyone can be saved.” Potentially making him a bit darker. He also admits that Tyrande was right and vows to listen more closely to her wisdom.
Tyrande: Teldrassil. All I really have to say on that.
Malfurion: Again. Teldrassil. Though, I’d say having him completely body Nathanos would be more satisfying to see? Because he’s always seemed more focused on the land… and Nathanos was the one leading in Darkshore after Sylvanas departed. (I also just kinda want to see Nathanos get ripped apart in the background while Tyrande goes after Sylvanas.)
Thrall: BECAUSE WHY NOT STEAL ANOTHER KILL FROM US, HM?! But, no. In all seriousness; He takes up a role as a protector of the world and the Horde (again.) He’s always been one about preserving honor, and it would be a neat way for him to get some of his power back, I feel. Confronted with his bad decision in a way that he can’t just blame someone else. This is a result of his initial mistake, after all. A weaker choice, but…
Baine(+Jaina as a bonus): He needs a moment to shine. The tauren people were threatened by Sylvanas, and she has constantly affronted his sense of honor. But I feel as if it should be a team effort with Jaina if that’s the route… Baine rescued Jaina’s brother (who I can’t remember the name of) and she has just as much of a stake in this as anyone. While the grounds of their grief isn’t as great as Tyrande and Malfurion’s? It was more… direct. That’s family… annnnd personal torture.
Genn: He let her live. It could be a set up to right that mistake. Even while he was wounded in Legion, and he pretty much called it even after stopping her from enslaving the Valkyr that I can’t remember the name of… she stole another home from his people. Gilneans have always been proud, and Greymane was always taught to protect his kin.
That was going to be another. Being hounded, he just starts to break down. No one but his thoughts to keep him company, he’s forced to deal with the fact that he STOLE. HIS. NEPHEW’S. CORPSE. Things start to click that he’s done all these horrible things out of love and/or admiration for the Sylvanas he knew in life. He slowly comes to realize that she doesn’t particularly care for him any more than she would a favorite toy and that these are things he wouldn’t do otherwise… realizing that she’s too far gone and is nothing but a distant memory.
Chis Metzen: Just Metzen. No fancy look or anything, just in a Horde t-shirt with the Doomhammer in hand and the doofy helmet he used to use in his forum avatar.