I think all the blue people tribes should end up getting infiltrated by an espionage team of loyal humans (ung Avatars of course). Also, we should be introduced to a new threat that lives deep below the surface of the planet or the orbiting moon, too. We should explore a whole new world within the world that we are already familiar with. That way we can have something new to look at and go ooooh and ahhhh to. As for the romance stuff... well, the only way I can see that working is if you make it appear that either Jake Sully or his blue girl friend is dead (when really they are not). Perhaps the other person is about to move on with someone else... but right before the other one makes something milar to a marriage commitment... the other one shows up (which complicates things romantically). Either that or the other person ends up getting married and doesn't love the person that they are with or something.
Oh, and by the 3rd movie... the humans should eventually conquer the blue people and have them integrated into a new human nation on Pandora. Perhaps the humans get the blue people to have special new reserves or territories where they can use man's new technology or benefits for their own choong. Sort of like the tuation that the Native Americans have today (Special Indian Reserved Territories and the benefit of having Canos and stuff). However, we learn that although the blue people have made out somewhat (after letting themselves be conquered) they are now doomed to a world that is in grave danger environmentally. And maybe... just maybe some of them have come to realize that if they haven't been tricked by white man's ways... they could have continued to fight and maybe had a chance at having a natural safe world as they once remembered it.
Please do not take offense if you are Native American (Indian). I mean no disrespect. I am mply re-telling how things could have played out milar to our own history. If my wording seems to rub you the wrong way... please know it was not intended to cause any disrespect or offense whatsoever.