Honestly the only person I can see leading the orcs is Thrall. While he may not lead the horde directly his role of advisor will allow him to steer the horde through the rocky path that has been left in the wake of Garrosh's defeat. What does concern me though is how the alliance will steer itself as this conflict closes. While many in the horde are ready for peace, there are still some parties that are not exactly ready or willing to throw out the white flag of truce just yet. I would imagine with the next expansion we will see those same forces attempting to throw a coup now that they know they can eliminate one horde warchief, why not another.