(Avatar plot spoilers below)
Readers will be relieved to learn that the violent action sequences in Avatar do not need to be repeated in the two sequels that Cameron's been talking about.
It seems pretty obvious that as Cameron plans it, the humans will still want the unobtainium, and won't be caught by surprise as was the case this time with a small human expeditionary force facing the globally-united Na'vi. So the sequel will be a massive human attack on Pandora - call it, say, The Humans Strike Back. It will end somewhat inconclusively, until the third movie where the Na'vi leave their world, get into an outer space battle, unexpectedly convert one of the bad guys to their side and then win. Call that one the Return of the Na'vi.
That sequence is all unnecessary. The key issue is that Hometree, which formerly blocked human access to the largest amount of unobtainium in hundreds of miles, is now gone. Obviously it would be better to have been saved - riches of Pandora are at the surface and all that -but I've learned in my environmental career that once you've lost, you move on.
So rather than fight each other, I can provide a completely different plot for the two sequels. In the first sequel, we have a Star Trek Next Generation-style plot, where everyone sits around a table and conducts dramatic negotiations that if they break down, could lead to violence. Fortunately, Pi'kart of the Na'vi realizes that they can let the humans mine the unobtainium where Hometree used to be, without further harm to the moon's environment. They also allow mining at other already-disturbed sites, and they offer cooperation on learning about the planet's biological wealth. Everybody wins!!
In the last movie, the Na'vi return to their hunter-gatherer lifestyle. We've already seen the hunting side, so this movie should focus on gathering. I suggest two-and-a-half hours of watching the Na'vi dig out blueish tubers from the ground. The climax would be a fifteen-minute struggle to pull out an especially-tough tuber, with a team effort victoriously resulting in a slow-mo explosion of dirt as the tuber is finally wrenched out.
No need for Cameron or the fans to thank me for the plot fixes.