My buddy actually set up a RIFTS campaign for this. (Rifts is like D&D only it runs on a percentile dice system for most of the skills and such, rather than the D20 being used for nearly everything.) I know I'm a nerd...but he originally got the idea from Borderlands, which was A GREAT GAME.
Avatar was a good movie, sadly I heard the game was poop. (Because I'm getting tired of blotting out the word s**t and I'm wondering if I'll even get in trouble for saying it...)
In short...It would make a pretty dull world for an MMO. The setting is nice, but what would the races be...Alien and Human. What would the classes be? Gunner/Archer, Pilot/Archer, Infantry/Tribesman, Medic/Healer...it would need to be expanded on a bit.
Overall the game as of now would receive a Q...because I'm not a critic with enough respect to give out a letter grade.