I think they should drop the whole post-adolescence period for Aang. But if I have to say something, I think seeing an avatar in a modern society similar to the one we have now who is the opposite of Korra in personality and ideals who faces a great spiritual crisis that threatens the world and he must reverse the fusion of the physical and spiritual worlds to solve it, and regain his role as "the bridge between the worlds" again. Also that he regains his connection with his past lives.
