Most survival horror games tend to do with the supernatural. Resident Evil and Silent Hill are examples. However, I think that it might be scarier if there are no supernatural elements. Just reality. Now, what's the scariest place that can be seen in real life? War. In this game, you would be a civilian trying to escape a war-torn city. Just about anybody with a gun would fire on you, due to the heightened tension. Grab one if you want, but it would slow you down and make you a lot noisier. You'd also have the option of putting on a dead soldier's uniform, but soldiers might not be convinced of your disguise. If they seem suspicious of you, your best bet is to run.
The idea might need some tweaking, but I think it's pretty good. Also, what war should it should be set in? The Iraq War? Vietnam? World War II? Maybe a fictional war set Twenty Minutes Into The Future?
A better question is why I get my best ideas at one in the morning?