i think you should do player recolors so players can customize their car however they want to, and it'd save on you having to make a million versions of one vehicle. not sure how you might get the lights and sirens to work with it
an adjustable spotlight would be cool too, maybe have it work like the flashlight mod, except you press a key, it goes to camera mode on the spotlight, and the player selects what to shine a light on and then presses the same button to exit