If ever they're not pushing though with the legend of Korra, i have a book 4 idea: before the air nomads built the 4 temples, they were nomadic (travellers) , so they probably have some offspring somewhere. Aang and the others should go around the world searching for the offspring in order to revive the air nomads. The problem should be something like: The spirit world is changing and moving closer to the human world. However, the human world is off balance due to the lack of airbenders. If the two worlds come into contact while still off balance they will both be corrupted and destroyed!