The environment should be more interactive. If there are pine trees or oak trees on the side of the track, there should be pine needles/leaves accumulating on the track underneath them. When cars go by, the debris would get tossed around. If it is raining, those leaves would stick on the track and cause a slick spot. Also, if it is raining and you're directly behind another car and he goes off the track (two wheels or one) it will spit up mud/wet grass and stick on your windshield. If this happens a lot you're gonna have a bad time until you get to the pits to have it wiped off.
The crowds need lots of work (mainly talking about endurance here, but it could work for all). They should be more dynamic. Start/Finish should have the highest concentration of fans, if it's raining they should thin out and be using umbrellas. You should see random people walking around, kids on bikes, dads lifting kids on their shoulders, people that react to crashes and good passes.....
All of that sort of thing is not going to be possible on current gen hardware.