It is rumored that Stephen Lang will return as Quaritch, though he was killed in the end of Avatar. This may mean that the next film may have flashbacks, be a prequel, or has certain scenes from before the first film. It could also mean the RDA cloned Quaritch or built him a new body. In an interview, Lang says "It's not over while I've still got DNA" strongly hinting towards a possible resurrection of Quaritch. Recently it has been confirmed that Joel David Moore will return as Norm Spellman. At the Golden Globes, Sigourney Weaver hinted at the return of her character, Dr. Grace Augustine, in some form, saying, "If you look at the footage, I don't really... I go somewhere, right? It's science fiction. Anything can happen."
yes, it is a long explanation, but that should be enough.