Saurfang disobeyed a direct order and has been obviously unhappy with developments. Nathanos hesitated to fire at the world tree. She is losing support. Thrall needs to resurrect Vol'jin and we can take her down raid-style.This reminds me too much of Garrosh, though, and that entire rebooted Burning Crusade.
I am so sorry that Sylvanas isn't the perfect conqueror. she has a major flaw: underestimating the living. This is interesting.