Idc about the Horde turning against him. The Alliance should've been planning to wipe him off the face of Azeroth after Cataclysm, but didn't. Gilneas, Theramore, Barrens, Ashenvale, Stonetalon ya, these should've had the Alliance furious already, and they aren't. It's an amazing amount of sheer stupidity and fail on account of Horde loving lore writers that the Alliance needs yet another violent, depraved loss to really get mad at them. One the horde can "sympathise with" (who cares). Alliance can't get mad unless they are basically a corpse, but scratch the Horde and they are hopping pissed, and jumped up about Taurajo -.- (even in lore)
I think the "atrocity" was already committed, forcing his own people to get possessed by the Sha, which destroyed them all outright. But I guess that's not enough either.At worst I'd say Garrosh would try and use the Sha again but Cho Lorewalker tries to persuade him but Garrosh kills him in outright cold blood. That could be one reason for Pandaria to fear Garrosh's regime. It may not necessarily piss US off but it will annoy the Pandaren enough to stand up to Garrosh and thus maybe smack some sense in the Horde. I'm sure the plan will be much more grand but it's an idea I'd like to see if it was possible.