Small litle thing, we would love
Game breaking:
Modifying game campaign, eg. making 1980 ending year or making no one buy your cars in year 1964, 1984 etc.
Game building:
Adding new things to game campaign eg. let you go past year 2020 to discover technology of the future
Decorative:
Wipers, fuel caps etc. (Someone already said something about it)
Nonsense:
More colors and\or colored glass.
I know it's can be pretty hard, but everybody will love this.