As much as I despise the rice burner community, I have to agree. Part of the attraction of GT is the diversity of cars available. They would add to weight and drag, so most RACERS wouldnt add them, but it could be fun, and with online capabilities, you could develop an identity associated with your car.
Actually, they could add rice power to your car, and you could have races that involve standing next to your car, and people walk by and tell you which car looks faster.
Nitrous would be fun, but utterly useless unless there is a drag racing capacity. Drag racing gets old after a bit in real life, and REALLY QUICK in video games. I'd rather see other features added that drag.
You can get new rims since GT2. I am ashamed to say that I do in fact have special rims for some of my cars that I frequently use. They just look cooler.