I think it would be a good idea to have a mechanic shop where you can customize cars, and possibly a garage system that if you store a car in a garage in one part of the world, and find a garage else where ytou can grab the same car. This would make it so you could have a custom car with a more powerful weapon with storage slots that you can grab in a different location.
The garage could be attached to the mechanic shop or a stand alone thing