The Customisation in GTA V as a whole is pretty lacking. In my Opinion there NEEDS to be an addition to the Selection of options available.
Here's a few I think should be Added:
1.Lower suspension option.There's already a trick to slam your cars but why not just add it as an option?
2.An Option to add Camber to Vehicles
3.Widebody Body kits option for Cars
4.Hydraulics for Low riders.This is California for f*ck's sake R*
5. An option to Change the exhaust note on vehicles.Basically it would be the same thing as changing the horn.
6.The Option to add liveries to the car. For example adding the Atomic logo on the Sun strip or on the side.
7. Lift Kits for Off road trucks.
8.Bullet proof Glass for level 120+ Players. I'm not sure about this one as it may be too OP.
9.Chrome Rims need to happen.