Just a quick idea.
I like how the sports car works, you research the body style and then the convertible so I think something similar could work for the other models.
Example: The sedan could be further researched into a coupe and also a convertible; the SUV into a Van (not like a cargo van, more like a GM suburban or larger SUV) and the compact into a 3-door bul also into a station wagon.
Keep up the good work, you got an awesome game!