Cameron told Deadline:
“Steven was so memorable in the first film, we’re privileged to have him back,” Cameron said. “I’m not going to say exactly HOW we’re bringing him back, but it’s a science fiction story, after all. His character will evolve into really unexpected places across the arc of our new three-film saga. I really look forward to working with such a gifted actor, who’s also become a good friend.”
Sigourney Weaver has already said she’ll return, so it appears any major characters who died in the first film will be back one way or another. In the case of Dr. Grace Augustine her soul still exists in Eywa (the big tree) so we can picture how Cameron will bring her character back, but with Colonel Miles Quaritch it will probably involve something more complicated.
James Cameron and 20th Century Fox are planning to shoot the next three Avatar movies simultaneously, and have release dates set for December 2016, December 2017, and December 2018. Sam Worthington recently confirmed filming will begin October 2014, so there will be plenty of details and casting news released over the next year.