The strict rule of putting every car into just one category implies too many limitations in my opinion (in both Forza Horizon and Forza Motorsport series).
I’d suggest using tags instead of categories so that every car could be grouped with others based on various criteria. It’d give developers much more freedom in coming up with interesting events.
For example, 1965 Mini Cooper S is put into Classic Rally category in Forza Horizon, while Forza Motorsport 7 has it under Major Micros label. Why not combining both factors?
We could also see some categories split for more accuracy with tags. For example, Early Prototype Racers could be divided between Group C legends and prototypes from late 90s (like BMW V12). Or Group B cars separated from modern rallycross vehicles among Rally Monsters.