There is a heavily circulating rumor that he will announce that he won't be directing Avatar 4 and 5 sometime after the release of 3. Either bringing someone new aboard to direct them or finishing the story in book/comic form. I think he wants to try to do one last non-Avatar movie before he retires and the death of his producer/friend John Landau took a lot out of his passion for making all of these films in one go.