The Forsaken are one thing, but there is absolutely no reason the other races of the Horde, ESPECIALLY THE ORCS, would rally behind Sylvanas to wage war against the Alliance again after Garrosh. It just makes absolutely no fucking sense, especially in the wake of Legion craze.
Like how do you even justify this premise? What would Illidan think of the fact the Alliance and Horde suddenly decide to fight each other over the Legion?