3. Improved Car Customization
Every standard cars all will had body customization like premium. It also add Swap Engine feature and livery editor for all car.
This. There were plans for a livery editor in GT5, so hopefully PD just didn't have time to finish it and we'll have a nice one for GT6.
Engine/drivetrain swaps aren't terribly important to me, especially if the tuning was just broken down a bit and got more specific with engine parts, forced induction upgrades and tire sizes.