Wrathion probably has a hidden agenda, but I doubt openly taking over the Horde is part of it. Do you really think the Horde would accept a Black Dragon who prefers to go running around in human form as Warchief?
Realistically I think it's down to Vol'jin and Thrall at this point. Vol'jin is the leader of the revolution, so taking over afterwards is fairly probable. Thrall is a contender still because still can't be sure if Blizzard has the guts to have the Warchief not be an Orc.
Another possibility is that Vol'jin will become Warchief, and handle war and other political matters, while Thrall will become a sort of "High Farseer of the Horde" and handle spiritual matters without technically answering to Vol'jin.