I'd actually make it good. SHA-BOOSH!
Real answer:
More action sequences. The action sequences were one of the best parts of the film, but there was sadly less of that, and more of the two unlikeable dildos trying to make character development that just wasn't interesting.
No Sigourney Weaver or Michelle Rodriguez. Weaver is a great actress and all (Nuke the site from orbit! It's the only way to be sure.), but she was very mis-cast in this film. I felt like she was only there because she was also in Aliens, another Cameron film. Same goes for Michelle Rodriguez, except unlike Weaver, she's not a good actress, and her character was pretty much useless. So I'd re-cast Weaver's character, and get rid of Rodriguez's character.
Add elements that differentiate itself from it's borrowed elements. I mean, I don't mind a film that borrows heavily from something that was successful, only if they manage to present it in new and interesting ways. They half got it right, since they made some jaw-dropping special effects, but the writing was just plain bad. Not awful, but not Oscar-worthy (Which is odd seeing it was nominated for an Oscar). It just SCREAMED Dances With Wolves and Pochahontas without feeling like it was a different film. What I'd do is that, I'd change some small elements that made a big difference, like the whole "nature" thing. I mean, they were almost the exact same thing as Native americans that it wasn't funny, and felt rather pathetic in how no thought was put into their beliefs. Why not just give them the same intentions as the prawns in District 9. They're simply misunderstood, yet unique. So it would just be about the humans fucking up the alien planet for the mineral, and they'd be destroying forests and shit, only this time, the aliens won't exactly act incredibly annoying and preachy, and won't be complete Native American rip-offs.
Make it more subtle. The movie is about as subtle as a train-wreck on top of an airport. I can see that the film was trying to make a point, which I can totally get, but it all felt soo preachy. For starters: The mineral. Unobtainium is really the best thing Cameron could come up with? The apparent "symbolism" is so. Fucking. Obvious. A five year-old can get it. Literally, my five-year old cousin was able to get it. Then there's the preachy "save the rainforests" garbage. I'd keep the message there, I guess, but tone down the preachiness by a landslide to make it tolerable.
And finally, get rid of the Ewokism. The fact that the humans with helicopters and shit were defeated by a bunch of blue tribal people with spears and arrows really required a suspension of logic. How about having a scene where the Jake, Neytiri, and the Na'Vi would steal a bunch of stuff from the enemies from an old abandoned base that contained old out-dated guns and mech-suits. That way, the Na'vi would still have a fighting chance, and it'd be more realistic.
tl;dr - I'd change a lot of shit that didn't work. The. End.
