Because some of us think we already got two good ones.Oh yeah, that's why Cameron ramped up the 4th one... because they can end it after the 3rd one.
So why do we need three 3+ hours films BEFORE the "good one"?
Whatever he does for the fourth one I don't think will be *that* crazy though. He's known for technical innovations but not so much when it comes to story. So I'm sure whatever "crazy" thing happens in the movie will have been done in some other movie that we're already familiar with or at the very least some sci-fi novel that he read as a kid. And I'm fine with that as it isn't too different than OT era Lucas' approach.
That's why I'm thinking that Na'vi coming to Earth might be on the table, it was strongly implied that that's what Alien 3 was going to be about so maybe Cameron will decide to just do it with Na'vi (as you said he's already borrowing heavily from the ALIEN franchise as it is.) I'm guessing the ultimate resolution will end up along the lines of Matrix Revolutions with neither side defeating the other and both agreeing to live in harmony.
Kiri seems like she's being primed to be the Chosen One/Neo so I won't be shocked if she ends up being the one to unite both sides.