I think it'd be awesome to be able to customize makeshift weaponry for your vehicles in the game.
Like weld some armor plates main body of the vehicular particularly the doors. That'd be nice protection.
Some reinforced bumpers with spikes to slice up the zombies as you ram into them would be great!
Some open/closeable gun ports would allow the player to shoot out of the car and be protected.
They wouldn't overpowered since they'd still be damageable plus they'd take a lot of resources to make the protection for.