If you watch the Director's Cut there are scenes at the beginning which show habitat being restored and some captive bred animals being reintroduced into the wild. It's a little more hopeful than the cinematic version. Basically the Earth's environment is being restored, although it's likely to take generations. The company was always exploiting the several light years' distance from Earth because the public would never support what they were doing and neither would governments (at least not openly on the record). But way out there, the folks back home had no one else's word on what was happening but the company's.
The invasion and terraforming plot is the most obvious way to go. But the way they left off the original, with the Navi starting to use human tech, and with some of the scientists staying behind, I'd be surprised if there wasn't a more formidable challenge awaiting them on Pandora this time around. The Navi are more attached to the environment and wouldn't wreck their planet the way humans did, but I'd expect orbital facilities and probably some ships to be there. And their ground forces would be much better equipped. More than just poison darts to worry about now.
There are lots of other things they could explore. How well are the humans in artificial bodies really accepted? There's a little human DNA in those avatar bodies. And since they are artificial, do they age normally or live indefinitely? And if they have children, are genetic modifications passed down to them? How well would the "half-breeds" fit in? There's sure to be disagreement among the Navi about how many human innovations to adopt in their everyday lives. Some villages will likely take more to the fancy toys than others. Some tribes continuing to live in the forest and spurn too much technology, others starting to build human style cities. That'll create some tension. There may also be a faction that believes the only safety for their race is to take the fight to humanity, attack and subdue Earth. I still don't know how you get four movies out of all this. Hope James Cameron knows what he's doing!
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