It's a horror RPG! My main inspirational material is fun mid-century stuff. Anything from The Blob to spooky mansions, the Wolf Man, Dracula or weirder. In the world of RPGs, I think of Chill 1e/2e as right in the wheelhouse, but also the core nWoD book. The RPG inspiration that is hitting me even more right now is GDW's Dark Conspiracy. While it is set in a strange, particular setting, a lot of the material seems like it would also make sense for a more generic horror thing. I also like the careers, though the idea would take some tweaking. You have the mundane careers you see in Dark Conspiracy (or even Traveller: TNE), or you think of careers ala Warhammer Fantasy Roleplay, or the silly "careers" we dreamed up for some silly game just post-HS. Then I have this d10 mechanic I'm liking that seems pretty adaptable to the GDW assumptions, but are different. So, a core mechanic, some careers that evoke normal folks and/or tropes. I'm also thinking characters need some kinds of traits or passions that could possibly lead them astray. Maybe they could even have lil abilities to give them hope that fit with genre tropes, akin to a hack a friend of mine did for a slasher game we played a couple times. Also, never forget the advice from this "HOWSCARUM: A Primer for Running Horror RPGs" article from Chocolate Hammer. It's great! Oh, and another factor: A lot of RPGs have "advancement", and this is not necessarily a feature that models many stories in the horror genre. Maybe something that allows for something like advancement, but it could be advancement in a more normal career, or it could be advancement in a way that makes you better at dealing with the supernatural, but at a cost (you are basically more weird to normal folks, etc.) Or, always thinking back to Golden Heroes and its campaign ratings and Daily Utility Phases (and MSH and its Karma in some ways), it is a struggle or matter of balancing priorities somehow. Even Blades in the Dark downtime activities might have something to offer here. Exposure to the supernatural creates stress, and that stress can mess up your normal life. You can cope with stress in more or less healthy ways, or it may develop into more permanent neuroses.
Think it up!