I would love to see drastically expanded
"Ownership/Maintenance" aspect in various directions of GT Auto store.
First, I would love introduction of more progressive (scaled) curve of damaging the vehicle (engine and chassis). The current (GT5) is very linear (car is either damaged or not). I would love to see some progression (phases) that would be repercussions of driver's style of driving. Which leads to..
Detecting the damage - introduction of Vehicle Diagnostics/Dyno. In order to determine the level of damage on your vehicle, you have to submit it to Vehicle Diagnostics. There (with some currency to be paid) you can observe the current status of particular vehicle. Depending of results, you can submit your vehicle to either Engine Rebuild or Chassis Repair processes. In the same proces, the Dyno provides you with up-to-date data of the current engine power, etc.
How does damage occurs? Game tracks the way you handle the car (crashing, bumping, off-track riding, etc.) and applies some level of progressive mechanical damage to the vehicle. More you take care of the car, less damage it takes. Of course, mileage itslef adds its own damage (as it does in GT5) which at some point results with "unrefreshable" vehicle. For engine damage game tracks the way you use revs, shifting, etc. and acts accordingly. Of course, if you drive with "old oil" the damage is faster. Which leads to..
Days of Delivery
My favourite part of my idea. So, I basically give more importance to Delivery Truck. Instead of everything being finished within "1-click" I would also gave back more "meaning" to in-game days (which still exist in GT5, but they are not visible except on game-save data). When you submit your car to particular procedure, that procedure takes some in-game days to occur. So, for example:
same-day procedures (owner stays in car he brought to GT Auto): Car Wash, Oil Change, Dyno, Rim Change
1-day procedures (owner must take another car from Garage when submitting car to procedure, car is back in Delivery Truck in the next game-day): Vehicle Diagnostics, Spoiler attachments, Rim Paint
3-day procedure: Car Paint, Engine Rebuild
5-day procedure: Chassis Refresh
You can also multi-select the procedures in which case it takes the maximum days of "longest" procedure as overall time, it not adds up.
Of course, it would be more then great that actual parts of the car are experiencing wear due to ways you're using the vehicle (especially suspension and gearbox parts) so you have to re-purchase them and install them during the life-cycle of particular vehicle. But I guess many players would not welcome such miniscule attention to detail. Pity.