Though extensive bodywork is cyberpunk-esque (damn rice burners), I'd also like to see upgrades regarding functionality of a vehicle. Different grades of engine for top speeds, different size gas tank (5,10,20,30)L. These figures are much smaller than a real world gas tank because they revolve around what I understand the in game tank capacities to be. If fuel efficiency is factored in a 5L tank should be adequate for a cheap motorcycle. 10L would be for the economically minded. 20L is the standard. And 30L tanks are for gas guzzlers. However for desert folk external upgrades are yet another option to add another 10-20L on top of their 30L tank. External tanks would obviously be visible from the outside which will probably make you the target for a hefty number of parking and speeding tickets.
To prevent everybody from getting external tanks there need to be trade offs. Maybe it should cost a pretty penny, or have safety implications, or reduce fuel efficiency. But lets not stop at fuel tanks and engines. Give me mufflers, transmission, stereos, radios, on board computers, and a damn glove compartment!