I could see Mark Hamill as the mentor training a new Jedi Knight, putting him in the Qui-Gon/Old Obi-Wan position from the first installment of the previous trilogies. Kill Old Luke in Episode VII and bring back his ghost for VIII and IX. It would be fitting--and Jedi Luke dying could be powerful if handled correctly.
luke won't die as he is the "subliminal" Savior..he'll be in all 3 and live