The Last Jedi changed a LOT of things and one of those things was the way Jedi ghosts are perceived and can be used in these films. Yoda had a brief appearance in The Last Jedi as he had a chat with his one-time pupil Luke Skywalker to snap him out of his misery. But it wasn’t Yoda’s sudden arrival that was impressive. It was the fact that Yoda, as a ghost, was able to summon lightning and to knock Luke in the forehead with his stick. Up until The Last Jedi, it seemed that ghosts were only able to interact with people through talking. It was never apparent that they could affect the outside world or the people in that world which could prove very interesting in Episode IX.
As we all know, Luke became one with the Force at the end of The Last Jedi which left fans wondering if he would return for Episode IX. Well, he is and that can mean a wide possibility of things.