Exactly what I thought! Or you could have emergency vehicles.
Normal car = As you have alreadyTruck/school bus = Bigger in size
Police car = Goes fast with lights on
Firetruck = Goes fast and is bigger
Maybe people even crossing the road in front of you as the level of difficulty increases so you have to dodge obstacles both behind and in front. I think there's a lot of good ideas that could be put into this game.
All in all though, really cool idea that if you had more time I think could have become really great and a classic arcade type game.