[alfettish]

Sounds like a great idea.
There are some drawbacks:
It's been basically done with Gr. B rally back in the 1980s and, although in my opinion it was the greatest era of motorsport, it was very dangerous and short-lived.
Talking about a road race though, now that would be awesome. With modern car safety and a more organised and less-chaotic approach to safety, BRING IT ON!
Also look at the isle of man TT. Not unlimited classes, but quite a dangerous course.