So at the moment there is no real risk of cars. you can put on out unlocked and let anyone use it then just /getallcars for it easily. but for some challenge and such there should be a craft-able car lockpick that when used takes quite a long time and makes a racket but once done the ownership becomes yours, or even non-owned. and everything inside the car can be yours too. it will stop cars being layed around everywhere.
Also for extra features:
a locked car should take twice the time to picklock where as an unlocked car can be stolen quicker!!