I bet we're in for a Titanic-like ending to Avatar 3. In that film--not #2--I bet Jake and Neytiri both die, but in vastly different ways. I'll put my money on Jake being killed in battle late in the final film. As for Neytiri, she'll live for many years after, eventually dying of old age. Just as Jake narrated the prologue in the first film, she'll narrate a sort of epilogue in which we'll see the Omaticaya restored to their former glory, having had no human contact for most of a generation. Then Neytiri will go off alone and bond with one of the spirit-trees. Hearing Jake's voice calling to her, she'll lie down and simply "go to sleep" peacefully, finally joining him after a long separation.
That's the sort of thing *I* expect to see anyway. No brutal end for Neytiri, but an end nonetheless.

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