I'll start with the question I'm asking everyone:
do you think Earth and the humans that live there should be shown in any of the sequels?
I'm not saying that I don't want to see Pandora in the sequels; being able to explore more of that fantastic world is one thing that will excite me greatly! However, I think I'd also like to see a positive change happen on Earth, either in its biosphere or in its people. I would like a resolution to the problem of Earth's dying condition. And for that to occur, I think there needs to be some sort of interaction between Jake/the Na'vi and humanity as a whole. After all, James Cameron has said that the Na'vi represent "our higher selves, or our aspirational selves, what we would like to think we are;" wouldn't it be great if humanity in its darkest days could be touched and talked to by its aspirational self?
This article has a few ideas for a sequel that I really like. One is about using the technology left behind on Pandora to foster rebellion against the RDA on Earth; another is about the Na'vi producing art and other things that change and inspire the humans on Earth. (I have this vision of Jake and/or others in a video addressing humanity at large, and crowds of people watching it in places likes Times Square and Piccadilly Circus.) If the sequels show the future people of Earth changing their ways for the better, then I think that would give hope and inspiration to the present-day audience watching the movies. Another idea I really like was mentioned in another thread here - planting an atokirina in the earthen ground, and that being the end of the series.
...or Cameron could show Earth completely destroyed and humanity becoming extinct on its surface, just to say that we really are screwed after all. But I think there's potential for optimism nurtured by help from "our higher selves."
Hope I said all that clearly enough, what do you think?