Some things I have been thinking of that might improve the game:
Fixture design for mirrors
Fixture design for windows, meaning that a person can mold the windows of the car to his or her liking. For example, on a new Honda Odyssey, the back side window is shaped upward, a design a person cannot match in the game.
Individual paint sections: Some cars have more than one tone of paint (the bottom is a different color, or there is a racing strip, etc.)
Model Year editing slider: Much like how the old engine designer removed parts that were not invented during a preferred year, the fixture designer could be able to remove any designs that might not fit with a person’s desired period.
Hope these are somewhat helpful!