I have no problem with Sylvanas at all, but she is literally the least likely to have leadership of the horde if something is to happen to the current Warchief/Leader, so I can't imagine this wasn't a hostile take over. Currently she has the biggest army among the horde so it seems more then plausible but I can't help but think that's another Garrosh scenario waiting to happen (...ugh) so I'm confused as to why we don't have Baine and Lor'themar taking the spot.They nicely set up Lor'themar as a contender for the spot in MoP(...although he did try to leave the horde, only because Garrosh sacrificed his people needlessly) and Baine certainly isn't a bad leader and he his wise as well.
Maybe they die? I know Lor'themar is present in the Broken Shore cinematic
Either way I can't wait to see how this all plays out