How about enemies that you can *only* see when it's dark, and then not very well. Like, if you go with the light/darl, breakable lights mechanic, force the player to cut out the lights in order to see these weird, dim, shuffling forms, and then in later levels have these encounters followed up by regular enemies.
Also, include some sort of defence level. I love zombie defence.
Give the player some sort of relaistic motivation, like trying to reach a safe house or safe spot. Even if you then take away this place that s/he's been trying to reach the whole game once you get there.
Towards the beginning of the game, have Max Payne style tv's scattered about, givign you updates and information as to how the zombie menace is affecting the rest of the world, but as the fighting gets more intense for you, make it so for the rest of the world, untill eventually all outside contact is shut off, and you don't know what's happening.
Well, there's a mix of ideas there, for a more sort of 'haunted area' type game to the global Z-Day Apocalypse. Feel free the use any. But I want a creative consultant credit.