1) Slightly lower the cost of the economy cars to present an alternative to bikes
2) Reduce their item storage capacity if you do that
3) Make it possible for people to at least superficially damage a car with a weapon while someone is in it (but not when it's just parked on the side of the road, to prevent abuse)
4) Make it possible to shoot at the driver or passengers unless there is bulletproof glass (and maybe this is not total protection)
5) Make it possible to drag someone out of a vehicle that is damaged badly enough (like through a shattered window or crushed door) assuming you are strong enough to do so
6) Remember to disable this on NPC-operated vehicles lol
7) Make traffic less onerous
8) Make bikes crash easier if you're avoiding traffic with them and make this a toggle. Make crashes more dangerous in general
9) Make cars get dirty if left outside and give mechanics a car wash at their garages
10) Make certain parts (engine, transmission, tires) degrade with regular use in a way that affects fuel efficiency and safety, but make it so that any decent mechanic can fix this kind of non-traumatic wear and tear
11) Do that thing where any baka can completely restore minor damage but you need a master to salvage a destroyed part.
This has been gone over elsewhere in several threads but I think any of it would lead to more vehicle drama and more work for the mechanics. Keeping vehicle-bashing to scenes where players were present would also keep it from overshadowing the vehicle theft skills.