The potential for Luke Skywalker to return in Episode IX as a Force Ghost is always there, and considering that’s the only way any of the original Star Wars trio could return, the odds of it happening are probably pretty good.
At this point, it’s anybody’s guess whether or not Mark Hamill actually will return to Star Wars.
If Luke does appear as a Force Ghost it likely won’t be a particularly large role, which is probably for the best for Mark Hamill if he’s afraid it’s just not going to be much fun anymore.