The logical expectable thing would be that Sylvanas burned the tree and the Alliance retaliated, because she's evil and Horde right? Except not. They're gonna switch it and make the Alliance make the first move, I'm sure of this. Sylvanas will burn the tree after she fled from the EK to Kalimdor.
We already had a warchief as an expansion last boss, I hope we're not getting another one...