Being a horror movie fan, especially of 80s horror with it's iconic franchises and such, I would want to create a sandbox action-RPG game based around the idea of designing and playing your own movie-style maniac, with a variety of powers letting you build a fairly normal slasher, or a powerful supernatural being.
I'd probably go for an AO rating, by putting graphic sex, rape, mutilation, and following through on all of the exploitive stuff that makes shock horror great.
I even have a storyline in mind to form a campaign out of, ideas for side activities and side quests, and also a system based on things like the old real world VICAP system and my old criminal justice education to determine the response of authorities to your murders and such when your not simply on a rampage. The idea being that you could be a subtle monster and pick people off to rape/torture/kill them, or a Jason Vorhees inspired murder machine which might occasionally stomp through the local campground killing everything in it's path.
I see it as doing for horror movies, what the godzilla/giant monster games (Rampage, etc..) did for that genere.
Getting past some RPG ideas which may or may not sell, chances are that would be my pet project if I had the resources.
I'd probably call it Murderville, Maniac City, or Horrorverse.
The idea wouldn't just be to create a massive orgy of depravity, though it would certainly be that, but to create something really for the horror fans. The guys who can sit down and watch a "Saw" or "Hellraiser" marathon, or talk about classic FX heavy cinema, and how they don't do bad guys like Freddy Kruegar anymore.