Hot on the heels of the AH-nuld rumours Cameron just confirmed it would be stephen lang again.
"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."
On top of that, he mentioned a "new three film saga", further pointing that this is a sequel trilogy we are looking at rather than two sequels and a prequel.
Edit: Zoe and Sam have been confirmed too: http://uk.eonline.com/news/499570/zoe-saldana-and-sam-worthington-sign-on-for-three-avatar-sequels