Given various roof, front and back slots-
*Ability to get a tire rack/attach tires to the slot for easy access in case of an accident.
*Trunks, Racks, similar items that can hold items but be accessed from outside the car by those with the requisite skills. Would be nice for carrying more items in a vehicle for various reasons but give a downside.
* Buyable winches or tow truck attachments - allows people to haul hulks, etc.
*Motorcycle racks - A rack that would allow the storage of certain motorcycle or unpowered vehicles on the back of large enough vehicles for transport. (Unsure if this would break code).
Can't wait to see what comes down the line with vehicles, just some stuff I saw I wanted to bring up.