This could be a very broad topic covering a lot of shows. If you're able to narrow it down at all, that might help yield useful examples for your purposes. For example: does any play involving a flashback scene containing character/s that are now (in the present-day scenes) dead, count? Does it only count if that flashback is used to illuminate the psychology of the character remembering (this would probably exclude Phantom of the Opera; sorry, Old Raoul)? Do literal ghosts (Ghost: The Musical), fake ghosts (the Phantom in Phantom of the Opera) and 'psychological' ghosts (Fun Home) all count? How would you say that your initial examples used ghosts effectively or ineffectively, to give more of an idea of what you're looking for? (I don't mean for this to come off as a rant; I'm just interested in understanding.)
All that said, here are a few examples that I find interesting, if they're any help:
Elisabeth - Set in the underworld. The narrator resurrects a bunch of dead characters and makes them play out the story of what they did while alive, according to his own biased perception of what occurred. (There's also a character who's the personification of Death.)
Rebecca - Sort of an almost-ghost story (see also: Jane Eyre). A woman marries a widower and becomes obsessed with his dead first wife, Rebecca, whom she never met but whose presence seemingly lingers everywhere. Her preoccupation is so strong that Rebecca might as well be present as a ghost (and maybe she is... or is she?). However, I'm not sure if the show itself tackles this particularly effectively; it's inherent (and effective) in the original Daphne du Maurier novel.
Next to Normal - I have mixed thoughts on this show, but I think the use of a teenage Gabe as the 'ghost' of who he might have grown up to become if he hadn't passed away as an infant, is interesting. Given that he's the delusion of another character, Diana, rather than a literal ghost, he's also a menacing presence at times, seemingly trying to actively sabotage Diana's mental health.
