I woke up and had a pretty nifty idea: A horror game where all the enemies are completely invisible.
I'm thinking that it'd take place in a very ordinary domestic-style home or neighborhood, but your character is totally alone to explore it as they please. Atleast, on the surface everything is fine, there's no evidence anywhere of tragedy or wrongdoing, and you are alone... until you start hearing things moving things walking around, opening and closing doors, and then your character gets attacked for the first time from an assailant that can't be seen at all. I would have to design the game to advertise that headphones with the volume cranked up is necessary, as the enemies would have "perfect" invisibility, unable to ever be seen, even with tricks like throwing flour into the air. The mystery of who they are or what they 'look' like will never be revealed to the player, not even if one is killed, their invisible corpse simply ceases to exist, like fighting angry ghosts.
I wouldn't be satisfied with rote enemy behavior, I'd like to have the enemies behave in highly malicious ways. Striking once and then retreating, moving in erratic ways that just listening for them can't easily account for, trying to only move when you move to cover their own sounds, remaining perfectly still and waiting in ambush, always trying to attack from behind or when you otherwise can't defend yourself, etcetera.
The goal of the game would be to induce a constant sense of paranoia, to transform the ordinary domestic living space your character inhabits into a terrifying place where danger could be anywhere, could be following you, could be lying in wait to ambush you, could be mere inches away and is simply making no noise to alert you.
I suppose the game would be the extension that the greatest fear is the fear of the unknown, and that would be the premise of the antagonistic entities in the game, to be unknown and unknowable.