I was thinking about this...Since it is not confirmed that Garrosh will be the warchief of the current horde, but more then likely becomes warchief of "The New Horde", Whos to say Garrosh doesn't become a Semi bad guy, or a new NPC only faction.
Kind of a "I can do it better then you" horde, since thats what Garrosh is known for, I do not believe everyone will leave Thrall's side, guardian or not, he would still be in charge of the Horde.
I see him giving command to Garrosh while he is away and when he comes back Garrosh doesn't want to give it back, and splits the faction in half pretty much and thats why garrosh kills Carine, because he sides with Thrall...I would like this...Horde vs. New Horde mmmm