In the Special Edition version of Star Wars: Return of the Jedi, we see Anakin Skywalker appear as his younger self before he turned to the Dark Side and became Darth Vader.
The idea that Luke Skywalker would return in Episode IX as a Force Ghost is certainly nothing new.
What’s interesting, is that if Luke does appear as a ghost, it appears he will be clean shaven.
If a Jedi has the power to choose how they appear, perhaps Luke will choose to appear as he looked long ago as well, before he made the wrong decision with Ben Solo.