I know it's not the meat of the game, but having some small critters and insects in the game, especially in the outdoor areas would liven up the game and give it some ambiance.
The only animals there are now really are a few big beasts standing around looking bored, and a few other critters.
I realize these things are not #1 on anyone's list of items to add to a game, but they will make the game more enjoyable overall.
How about adding a few flies around the trash heaps or some butterflies or bees around the plants?
A few rodents in the slum areas of cities. etc etc.