Okay, so WoWwiki is a pretty legit source imo
It was revealed in the March 2012 Mists of Pandaria Press Event that Garrosh will be the final boss for the Mists of Pandaria expansion, as both factions recognize his growing instability and lust for power.[10] This means a new Warchief will be appointed and most assume Thrall will return, but Zarhym has hinted that the new Warchief may not be who everyone expects.[11]
http://www.wowwiki.com/Garrosh_Hellscream
Any thoughts/more sources?