I reckon Avatar could expand into so more. A novel. An MMORPG. But what would it be about?
Novel:
One based on the movie is a must. I'd love to get a book about the movie. But what other novels could arise? The Aftermath of the Human's eviction? Pandora's past or Future? A story based through the eyes of a Human, Avatar or Na'vi? (A human/Nav'i might be one-sided, while an Avatar is on both sides.) The possiblities are endless.
MMORPG:
I've played the PS3 version. Not best game-play, but I loved the graphics. Could it evolve into an MMORPG? I hope so... With the right bits and pieces, it would work. What would the classes be? Before, during or after the events of the movie? Would it rival World of Warcraft? It could only just be the start.