Reading the Dead Space thread gave me an idea. Since there's no EVIL DEVELOPERS here to steal our ideas, let's post any concepts that pop into our heads.
Here's my idea for a REALLY scary game: A game where you wake up unarmed, deep within in the Catacombs of Paris or a similar area, with a flashlight and ten hours worth of batteries. You have to find your way out, whether your batteries run out or not. Think about having to stop to change them every so often, especially if you just saw something move. It could be black-and-white and grainy, to enforce the sense of darkness and to emulate the night-vision on cameras.
That'd be terrifying.