It doesn't necessarily have to be pitched as horror. For example, imagine that the ghosts of Revolutionary or Civil War spymasters stayed around, organizing ghosts, using the skills gained in life, keeping track of the living who did wrong to the dead, aiding them in getting revenge or otherwise achieving goals in ways that didn't set off the mystically-minded among the living. A series of casual hauntings might alert a more responsible Magus, or other sorcerers. A bunch of coincidental accidents, on the other hand... Put this way, it works more like an espionage story, but where the participants are literally ghosts, can go anywhere, see anything, infiltrate any security system - the ultimate spy. They're under enough rules (you can't pass through a doorway with the right symbols on them, or whatever) that they can't just take over the world. Some factions might actually be interested in cooperating with the living, plugging into their intelligence apparatus or other assets. Others will be like "pff, we don't need them", and treat the Scooby Gang as meddling interlopers who need to be dealt with.