I vote for

1. Brake fade
2. Engine overheating when overrevved or when rad damaged in crash.
3. Complete engine blowing (with boom and smoke) when overheated for too long, when surpassing a certain RPM (eg. misshifting), or after a heavy crash.

ALSO...option to TURN OFF damage in single player mode...

I would also like to see touring cars which use road tires (like the Speed world challenge)

Sports sedans eg. BMW 3 series M3 and M5- Normal Aspiration important- to run with the current turbo's XRT, RB4 and FX0.

A car with a V8...like a muscle car for dragging, but with proper setup can be made to handle some corners.

Rally car (WRC or Paris Dakar type) for Rallycross.