For locations, we should certainly have a green room, backstage, main stage, and general seating area. Other places can include bathrooms, a lobby, orchestra pit, and catwalks.
I love the bathroom ghost idea. We should totally run with that. Maybe the ghost gives hints at who the murderer is? I could even make handwriting-style graphics to accompany that.
As with most theater lore, there should be some sort of word that you cannot say when you're inside (Scottish play anyone?) for fear of bad luck befalling you. Perhaps if someone says it, the post is edited to remove the word or the post is removed altogether.
Ummm... running low on ideas here. What if there's a rumor that there's a secret, boarded-up door hidden within the theater? It originally led somewhere but after multiple renovations it's said to conceal only a brick wall (a la Coraline). Yet, those unfortunate enough to find it are never seen again? I don't know.
JUST TOSSING IDEAS AROUND LOLOLOL.