One theory that I've seen thrown around a lot is that Garrosh is going to kill Thrall to anger the orcs. Considering that Garrosh is the antithesis of Thrall, and the orcs were more than happy to go along for Garrosh's joy ride, I'm going to bet that Thrall's death wouldn't anger as many people as we think it would.
I think it would have to be another war hero whom Garrosh killed for shock and awe, so my bet is going to Garrosh and Thrall having a stare down before Saurfang shows up, honorably challenges Garrosh, and dies in combat. Bonus points if its an underhanded death.