My Game idea? I actually have a big, long thing written out for if I ever get a chance to propose my idea to a company, but the short answer? I always wanted to do a game about being the movie monster out of Horror/Slasher flicks. I'll do a short rundown. You'd make your own character based along one of three types:
Brute: Your typical, run of the mill, unstoppable killing machine. The focus is on intimidation and surprise, with lots of muscle to really mess people up when you manifest (more on that later). Think Jason Vorhees, Michael Myers, or the Alien in Alien.
Spook: You are that which bumps in the night. The focus is on illusions and possessions, so your goal is more about messing with someone's head and killing them by fright than ripping them apart. Think Samara from The Ring or Freddy Kruger.
Manipulator: You literally change the environment into something horrifying, but it's no illusion. The focus here is on manipulating objects, turning the environment into a hellish landscape, or by making them witness things that should not be... in short, your trying to drive them mad. Think Alyssa (or the Town itself) from Silent Hill, or anything from the Cthluhu Mythos.
The actual gameplay is divided into three stages as well:
Stage 1 - Stalk : During this part of the game, your insubstantial and invisable, while you travel to a location to cook up some scares. You basically move around the world, looking for a likely place to gather Fear (which acts as XP to improve your skills, as well as unlock new places to haunt).
Stage 2 - Haunt : Once you have found some victims, you start using your skills to scare them. Your not overtly attacking them... you just trying to build up the Atmosphere of the scene until you've scared them enough to reach the Climax!
Stage 3 - Manifest: In the final phase, you manifest physical and start to unleash your true powers on your unsuspecting victims. Depending on your type, you can ether kill them through violence, scare them into submission/death, or reduce them to jibbering lunatics. Depending on how well you do, you get Fear, with more points being awarded for truely spectacular or cinematic final scenes.
I imagine the player starting out with horny teenagers/children, working up to guys like paranormal experts and priests, and then finally tackling some real threats like Ghost Busters, Final Girls, or Army of Darkness wannabees. I also imagine it as being somewhat sandboxy, with it taking place in a small town.