Well this idea will increase RP, and Decrease Auto stealing.
Basicly it means that there are shop/s, that there you can Dupplicate your car/s keys. (for cash).
Then for each car you will be able to buy extra key/s or maybe keys for all cars together (i mean, 1 key for a car, or 1 key for all cars, i belive 2nd
is easier to script), and you will be able to give the keys, this way the person with the keys will be able to use your cars too, lock/unlock them , etc.
this is good for RP, Car Renting, Handing a car to a friend, Sharing Cars, etc...