One of the things I'd like to see is a complete revamp and streamlining of the GTAuto/Tuning function. Anything related to modifying the car or maintenance should be under one roof with one loading time and the ability to skip any and all animations that may come with it.
For example, any aero or cosmetic parts, wheels, etc.,functioning or not, should have a picture of the car on the screen and a pop-up menu for cycling through the parts and doing a preview. Click the part on the pop up, it appears on your car. You can add or remove parts easily to get the look you want, hit one button and they are all installed at once and paid for. Hit another button and all parts are removed and you can start over.
When adding performance parts it should work the same. Car is on one side of the screen, pop up menu on the other side. You go through a checklist of parts you want to add, it gives you a running total of price and peformance adjustments, and you hit one button and they are all installed at once and paid for.
The thought of going through unnecessarily cumbersome and poorly designed menus, long animations, loading times, and checkboxes as in GT5 will turn me off doing a lot of car modifying in GT6 on all but my most favourite cars.