Well Yoda hit Luke on the forhead with his cane in TLJ signifying that force ghosts are some sort of physical beings, so Luke could come back and influence anything now…
Good point Dig.
And as that cane was real, then it’s possible for ‘Forceghost’ Luke to wield an actual lightsaber come Episode IX…so there’s nothing to stop him kicking ass with it either. And it’s possible for his sidekick Yoda to be able to do the same in that case…so we could get TWO green lightsabers taking on the bad 'uns if J.J. wishes it…
However, it’s more likely we’ll just get a brief ‘cameo’ of Luke giving a few ‘words of encouragement’ to Rey in the next instalment. 😉