in my eyes it wouldnt make any sense too pick someone else then Vol'jin as new warchief (well except prehaps thrall).
considering he is the one who starts the rebellion amongst the Horde races and all the others rally towards.
its true that other faction leaders already clashed with garrosh but they didnt turn on him till they had Vol'jin actually rising up agianst Garrosh.
therefore an other faction leader too become Warchief of Vol'jin would be weird.
the only other alternative i find appealing and actually preferable, would more be of a War Council.
though this may grind the Horde too an hold, since opions might greatly vary. especially between the Forsaken (and prehaps Blood Elves?) and orcs/trolls/tauren.