AI with analogue steering and brakes would be nice. GTR Evo still dissappoints in this area for me, especially as the AI all seem to charge around with ABS on even if you dont. I always thought the AI from the original GTR was tougher, harder and better so keeping that feel (I never had GTR2 so don't know how far into RACE territory it went) would be bliss. I'm also one of the few who like formation laps and pretty much the only one who likes the end-of-race in lap so an option to include or exclude those would be and easy way of pleasing everyone.
The ability to share a drive with an online partner where you can watch the race/telemetry and communicate with your team mate when not in the car would be nice and help to encourage more participation in endurance length online racing.
As for vehicles, I love Super GT but can't see it happening.