Mixing the features from GT2 and GT3 would be a great idea. Arranging the cars in order of specific design, such as FWD, turbo, etc. was a very handy feature and I hope it returns.
Including a parts bin for storing car parts for parts won, or for individual cars. Storing bought parts that can be taken to a race series would be awesome. That's if parts failure is an isssue in GT4.
The ability to work on your car while still in the garage. This is especially true if HP limited races return in GT4. It was a pain to deal with in GT2 when your car happened to be set up with too much HP. Entering a different race, or goin to the run setting track, was such a stupid pain.
If the before was true, then let us see the setup of the vehicle. not just the HP. Everything should be made available to us for checking and tweaking. Changing oil, camber, changing turbos, etc.
Since we will have user adjustable and selectable racing uniforms, helmets, gloves, etc. how about a closet for different uniforms for differnet cars, or teams, if racing teams are included in GT4.