Okay, so here’s the tl;dr. Cars will sometimes spawn without some/all doors, same with if windows are broken or not. Same with some other stuff.
Doors basically, finding a vehicles that the body is damaged but not the things needed for driving, adds to the apocalyptic feel. As well as cars having windows that are partially broken/completely destroyed. Likely, if the windows are only partially destroyed (half the window missing, or it’s badly cracked), it will probably just be easier to break out the remaining glass because the damage to the window makes it hard to see (this applies to windshields BTW. )
It would also help with immersion, having cars which look like the doors were ripped off or partially smashed windshields with blood on them?
Also, maybe instead of missing batteries in car sometimes, you have to take out a extremely damaged battery, or you have to jump-start a vehicle with another vehicle.
Possibly tires that instead of being completely destroyed just the rubber is gone and it leaves the (rim?) also, rubber should be able to catch on fire on vehicles, which sometimes, can cause a similar result to gta v (the vehicle explodes) or just the rubber burns off.
Any other ideas I have for this might be too complex. Though for fixing vehicles that spawn with a damaged engine you would have to have a few materials and your character would have to know how to fix the vehicle. Also, damaged vehicles should be more likely to have keys in them, since the person was probably killed after being dragged out. (maybe you could find keys for more important vehicles in places related to them, say a parked semi would have its keys in a nearby locked building or something.)
Also, cars should be missing other types of stuff, bumpers, lights, ect. And most likely you would be able to replace those with parts from the same type of vehicles (as in company and model) and replace those. So technically, you could get rainbow colored vehicles xd