1. A Secondary Antagonist
Stephen Lang has already been confirmed to reappear in the sequel as Colonel Quaritch, despite the fact he died pretty convincingly in the first Avatar. So hopefully the way they choose to bring him back will be credible enough as well as their reasons for bringing him back. But I'd also like to see another villain that poses a threat to Jake and his new tribe, in the same vein as The Walking Dead, which not only had zombies but also introduced the threat of other humans at Woodbury. Similar to my other speculations, this could be simply an extra factor that adds to the story or it could work in tandem with some of the other factors that Cameron has confirmed. Maybe a rogue Na'vi tribe that incites civil war or a biological threat that requires Jake to begrudgingly accept help from humans (Sigourney Weaver's scientist character is also confirmed to return from the dead - think about it). In any case, whatever form this villain may take, I think a secondary antagonist could work in the story's favour.
Anyway, these are just a few my ideas, some of which are based on rumours. What do you guys think? What else would you like to see when we return to Pandora in a few years time? Should Quaritch stay dead, or will it make for an interesting scene when he returns? Sound off in the comments section below.