Car customization isn't a necessity but I sure as hell would like to have it. I don't mean to a Forza or Gran Turismo extent but simple stuff, like Upgrade 1: faster Upgrade 2: Faster and better handling Upgrade 3: Further increases speed and handling etc...
This could be tied in with a mission for a garage (like Brucie in GTA 4) or a situation where you buy and run your own shop. Most of the stuff I want is just things that can be tied into missions to make them less of the 'Go here, shoot this guy or these people and escape the cops' type of missions
*Not to mention it would make a feeling of accomplishment after you complete the mission rather than just the cash you get