If MoP story was, like, adding up to something at all, most of the ideas here are quite odd.
The biggest expansion about peace, about compassion, about inner peace... and it boils down to revenge? Those the motives of most frequent bets here - Jaina and Vol'Jin. Even saw some votes on Baine, because of the fact Garrosh killed his father in a fair fight. If Garrosh REALLY has to be killed, there are 2 options:
1) It is done by somebody who does not have a personal grudge against Garrosh, and does it because of greater good, honor or heroism (Thrall, Saurfang, Eitrigg, Nazgrim, Baine, one of the Pandaren)
2) It's done because of revenge, but as an act of evil itself, and is either another seed of conflict, or a reason to villianise the killer himself (Vol'Jin, Jaina, Baine - if it's the daddy case, Varian, Tyrande)
3) Garrosh kills himself, either in a failed attempt to absorb even more demonic/sha/old god/whatfuckingever energies, or by a suicide during a sudden realising of what has he done.
PS: For me, Garrosh should end up remaining the leader of the Horde. Like, he is the last boss of Siege, but guilds struggle weeks, even months to kill him. After some time Blizzard answers the qq: "What? Sorry, Garrosh is unkillable. He is the Warchief of the Horde, duh. What the hell were you thinking?". Then the guild that got the closest gets a 7 day suspention and a one-way ticket to the Alliance if they are Horde, and everything remains the same.