Here's my 2 cents. Might be worth only 2 cents.
1) No PP limit. No matter what, anyone can change it. With this whole Hybrid stuff happening, anyone can do it.
2) Either allow all chassis swaps or none. There are only a few cars that can get away with this (efini FD w/ spirit R). This becomes a trust issue then. Do I trust anyone? Not one bit, but since you probably don't have the time or will to check every single car that enters, that'll have to be an honor thing.
3) Tires should be voted on. I personally like the feel of CM's, only because that's the same feel you get with Forza, which I'm familiar with.