The ability to weaponize vehicle doors would've been nice. Sleeping Dogs/State of Decay style. Open them up and just punt people as you drive by at ludicrous speeds. It would've been amazing with the GTA V engine.
Seems like something that would be relatively easy to implement, does it not? Could throw it in there in a patch. Make it a "weapon" that you can scroll to like you would with your SMG or pistol while in the vehicle.