On a side note, have you read the Metro 2033 book? If not then you really need to as it is leagues better then the games (which I also liked) and will really inspire you.
I read the book a few years ago. I know I read an english version so there may have been translation issues, but honestly I wasn't super impressed. When I think of rules to game it I think you would need motion rules that forced you to take hours to days to travel a couple hundred meters... Seemed to take them FOREVER to get anywhere.