i would say (and some of this may have been said, some by me before GT5 came out)
1. Tire and wheel sizes, width, offset, weight all of it plays a drastic role on the cars handling... im baffled this option is still MIA in GT.
2. A bit more JDM tuning, PLEASE, PD is from the motherland of car tuning, im sure they can source a few more body kits, spoilers, exhaust sounds and wheels.
3. A livery would be nice, but i personally woudlnt even use it much, even when i played forza it was "just there" for me, but adding stickers and being able to a bit more then just paint the car would be nice.
4. PLEASE KEEP THE "USED CAR LOT"... but No more standard cars, please. make them all fully modeled cars, even if there is going to be less cars in the game at least they all look up to PD standard, function well and are fully customizable and it would be a real treat to see some of the used cars from GT2 in full detail.