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.
It could be simply the result of their merging. Raava and Wan are one in the same. They are a single spirit (just with distinct personalities), they can't die... unless they enter the Avatar State, in which it seems that the Avatar Spirit (Raava and the other Avatars) live in the Spirit World until they are called upon and then physically possess the current Avatar. Similar to how Tui and La could die because they possessed the bodies of Koi fish or how the Spirits in Beginnings could be killed because they were in the physical world.
Which makes me think...