Well, I couldn't see it among the OP entries, so I'll suggest it. Apologies if it's already here:
Tire wear for endurance races needs to be fixed. In particular, the gap in wear-rates between Racing Softs, Racing Mediums, and Racing Hards needs to be increased. Therefore, RM tires--being less capable performance-wise than RS tires--should last at least twice as long. RH tires, being even less capable than RM tires, should last twice as long again. As it is in GT5, RS tires perform so well and last so long that it becomes pointless to attempt to minimise pitstops by using RH tires.