I think that A-SPEC, B-SPEC, the PaceCar, RUF, Lamborghini and the Land Rovers should stay. I also think that Land Rovers should be able to be bought in the dealership!
Hopefully, Porsche will also be added. It's a shame how Ferrari made it's way into GT, but not Porsche.
Online features should stay the same. There could also be a new feature that you can select if you want a male or female avatar. My sister has a hard time playing GT5 because the outfits are boyish and the avatar appears to be male.
In tracks, every fictious track must stay. I'd also like New York Circuit to come back to Gran Turismo.
And now for interior designs! Before buying a car, you select from a few interior designs. If the car does not have different interior designs to choose from in real life, then you cannot choose different designs.