I would find it interesting if you had a car, and it was YOUR car. Much like the horse in RDR, your car would spawn close to you after a cutscene or outside of your house. You could still steal cars, but, and maybe I'm reaching too far here, if you continued to drive a stolen car, the police, who are searching for a reportedly stolen car, would chase after you if they saw you (not too frequently). You could buy new cars, fix your car, or even sell your car if necessary. I think that'd be neat.