I haven't had a chance to play this proper yet, as it's hard to get us all together at once (mostly me being a graveyard shift).
But what if a combination with Pokemon? There are many Ghost Pokemon that are said to have been human before they were ghosts. That could bring in a couple of twists in how it's played. The type of ghost have a certain (possibly nonverbal) way of haunting as well as revealing what/who they are. It would also be fun to IDENTIFY the specific ghost type first and then piecing together why they spook the realm.
Or you could just leave it as no-human-only-pokemon. I'm still working it out with someone but I thought I would share it with others. See if anyone would want to brainstorm or even try it out!