Putting aside for a moment how butthurt people would get over a non-orc being in charge, I get the feeling Sylvanas may make a power grab in the chaos following the ousting of Garrosh. Her priority is to keep herself "alive" now and before cata Thrall had left the safety of Orgrimmar what? 3 or 4 times in total? I get the feeling being Dark Mistress of the Horde would put her fear of death at ease somewhat. There's also the matter of one of Lor'themar's subordinates telling him he would make a fine Warchief. I don't know if that was meant as foreshadowing or if he just wanted to pay respect to his leader but there it is.
Honestly this "who will be next Warchief" speculation is what I'm enjoying most about the story right now. If they just put Thrall back in charge I'll be pissed. Less because I don't want him as a leader and more because I want something unexpected to happen. I'm putting my money on Vol'jin right now but it feels like anything could happen.