In an interview with Den of Geek, Mark Hamill hints at a potential Luke cameo for Star Wars IX
"I thought: oh, okay, you should push my death off to the last one. That’s what I was hoping when I came back: no cameos and a run-of-the-trilogy contract. Did I get any of those things? Because as far as I’m concerned, the end of VII is really the beginning of VIII. I got one movie! They totally hornswoggled (tricked) me.’"
Hamill's "I got one movie!" is a decent enough indication that implies his role in IX will be minimal. I mean, they did kill off Luke in the previous chapter, albeit we all expected to come back maybe in hologram form, which might still happen with this “cameo,” or, as SW fans call them, “Force ghosts.” I presume this might have to do with the third lesson Rey never received during her time on Ahch-To.