Mr Light wrote:
Have they established that reincarnation is for people other than the Avatar? The Avatar only reincarnates because of Raava going into another person's body and retaining the memories of the previous hosts.
I don't think that's true. Just before Wan died, Raava said that she would stay with him through all his future lives. That implies that the reincarnation was a property of his own human spirit and she was tagging along for the ride.