Okay, Col. Quaritch is dead, and I can't imagine a good way of bringing him back.

Selfridge on the other hand:

Say that the people back on earth gets mad and fire him for retreating, and then he and a craw of the people sent back from pandora goes rouge, "steals" a ISV, returns to pandora to help the Na'vi to whitstand the new (presumably larger) threat from the humans.

Sure, it may sound a bit riddiculus that Selfridge manages to commander an ISV and then launch it to Pandora.

However I'm sura that Cameron said in a rewiev that he had though up a couple of sequel ideas since he wrote the story (first written ~1995). So hopefully it won't only be a sequel made just to earn money.