just a couple of suggestions for cars
indicaters and hazard lights
high/low beam toggle - better for night driving
open roof - can be used for sun roof or convertable soft top/hard top
What Bugs said. These are most important for cars imo. High/low beam, indicators would be nice too since you don't see them in any game. Open/close roof would be nice for convertibles although I could live without that. Also windscreen wipers (if this game will have changing weather this is be a must for sure).
Maybe some stuff for off-road vehicles could be predefined too, like tire pressure adjustment, low/high gear, diff lock (this would be also useful for street cars too, so you can toggle road/drift driving/racing). Now I don't know how realistic physics will be in this game, but if aim is simulator or at least close to simulation, than these would be great things to have, if not down right necessary.