I love how people say "Vol'jin's done nothing as warchief" when he JUST got the title, lore-wise. We haven't even seen him on the throne. Game wise, Garrosh is still the damn warchief and the chapter has not yet closed.
It's also ridiculous to say that Vol'jin, because he's not orc, has no part to play in the WoD storyline. Varian's not an orc. Think he's got a part to play in the storyline? But in the end, even if he doesn't go to Draenor, and he stays in Org, so what? He has a MASSIVE job to do: repairing the Horde. He has to get the races of the Horde re-unified. He has to repair and restore Orgrimmar. He has to rout the traitorous orcs left from Garrosh's influence. He has to maintain the Horde's military holdings in the face of Alliance superiority. Just because he's not on the front line of combat does not mean he's doing nothing.
We have not even seen the story yet. Maybe it'd be best to not speculate.