I've read (from some interview(s)) that Cameron had the idea for Avatar at some point in the 80's and has, since then, been developing the idea, predating the script's date obviously.

But, none of that matters since we all know Cameron was sent back from the future prior to principal photography for the original Terminator, so he would have known about all these stories already. Including that his Avatar movie would be a blockbuster success and spawn 3 sequels.