I would like to add, make the shuffle ratio "0" mean that everyone in the room has the same car. This would be a hugely beneficial mode for those who want to do spec racing in randomly chosen cars. Shuffle ratio "1" could be similar to GT5 ratio of 0, and increase car performance differences from there.
I also wish they had variation, not just "PP" levels. The random "same car" races noted above would be a great idea. I'd also like to perhaps pick a shuffle "type", like Super GT shuffle, or Le Mans Prototype shuffle.
Even better, "custom" shuffle for those hardcore people who want to choose their own cars to balance out the shuffle properly.