As for ingame effects I would personally go with:
In game:
Improvement to the smoke system/dirt sticking to glass with ability of wipers (apart from single seaters) to wipe it off. Damage improved, with the breaking of glass as mensioned above, oil spillage, broken glass can cause tire puncture. Tire makes to have different outcomes, i.e: Michelin/Dunlop have two different attributes.
Vehicles:
SUV, older modeled cars, something from the 50's as I have seen in rFactor, could have been 60/70's. Trucks is a maybe for all although would bring a whole lot more to the community as titles like 18:WOS are proving to be infamous. Finally just some add-on in-game player chosen improvements to the car like: Spoilers, wheels etc, not too NFS but more GT4. That would be all from me.