he's dead, but people high in the force can live forever in spirit.
Or clone themselves.
I wouldn't mind if they brought Palpatine back (ghost, clone or holocron) to warn someone about Yuuzhan Vong. Maybe have him warn a Jedi about them, while at the same time leading him towards the dark side. Maybe make this person Jacen Solo. Which would change the back story of the character some, but I think it would still fit him.