Unlikely concepts for the sequel.
1. The Na'vi turn out to be a relict of a extremely advanced civilization from thousands of years ago that had a kind of 'Unabomber' moment and decided that technological progress would bring only their own destruction. The Na'vi ancestors deliberately created a Neolithic Eden complete with a superbeing to control and monitor their idyllic lives.
2. Avatar takes place in the same universe as 'The Abyss'- the aliens in 'The Abyss' are from Pandora!
3. It was all a dream, Jake's still back in his cramped apartment on future Earth.