Sigourney Weaver says that she will be back for AVATAR 2 and from all she says it makes sense. If you have seen AVATAR then you know that there are reasons why she wouldn't be back, but there are also reasons why she COULD come back.
"Don't worry, I will be back," she said of Avatar 2, confirming the hint that she dropped nearly a year ago, "Jim [Cameron] says no one ever dies in science fiction. He's told me the stories for the next two movies and I have to say that they're absolutely wonderful and there's a real treat in store. Now we just have to make them."
Weaver also said that they have plans to shoot AVATAR 2 and 3 back to back starting next year which perfectly fits into what we currently know.
I am still going for the Jake talking to Grace at a Tree of Voices theory.