In a new interview with Comic Book, Hamill makes it clear that he’s done popping up in Star Wars as either a Force ghost or part of an unholy contraption that pisses in the face of God:

Mark Hamill cautioned that fans should not expect him to return as a world-weary Luke Skywalker in any forthcoming 'Star Wars' installments.

"We never expected it to become a permanent franchise and a part of pop culture like that. But my deal is, I had my time. I'm appreciative of that, but I really think they should focus on the future and all the new characters.