Black Flag: I am actually not to sure, but it still adds realism.
Tyre puncture: I was watching the old Le Mans on Motors TV when I saw the DBR9 get a puncture. It won't let the car spin, but will slow you down, the other drivers racing maybe when they get near the punctured driver, a warning will come up?
Pit crew: This could be a option that you could turn on and off, if you like seeing the pit crew, sorting out your car ok, but if you don't you could just turn them off, a bit like turning off the visablity of your wheel in the cockpit.
Lap timer LED bar: This could be in GTR cars which has a little bar showing your fastest lap, a bit like the ones of GTR and GTL.
Indicators: Ability to change the control of them!
Aids: The ability to turn on and off the traction control, Anti-Lock Brakes etc...