a little more realistic behavior of the traffic wouldn't hurt. I like how this improved in IV, but it still freaked my out how, at a red light, cab drivers always go nuts and try to push you over after a few seconds... but still way better than in SA.
and a more realistic damage model would be a blast. I said it in another thread already. different damage model for plastic and metal (must front/rear fenders are plastic nowadays), more parts that get ripped off (mirrors, hood, trunk, fenders, tires, doors, roofs, maybe even engines and axles) and a more realistic deformation of the car (after a while you can make a car look like a .zip folder) is what I would like to see.