A lot of interesting ideas, but....many have been tried before and not really worked out. First, the unlimited rules...while you would get insanely fast cars, the budgets to get there would be huge. If you think people wouldn't cheat the budgets if you put one on, think again. So, with that said, I would make one with an engine size rule, say 5 liters, spec the wheel base and overall width, and ban all ground effects. Let them have as much hp and mechanical grip they can achieve and set them off. I don't care if they go 280mph or so the straight as long as you have to brake heavily for the corners and slide through them.
Nascar was boring wiht a capital B when they had real cars. If you remember them from back then, it wasn't all that. It was pretty much like a 24hr race from a few years back when most of the cars broke and what were left were laps apart. Instead, I would make them run a fairly stock looking body that had to conform to an actual car. Make it tube frame still to keep costs down and make it still function and set up somewhat easily. (think late '70's and early '80's here) Nascar has been tube frame for longer than people realize due to the bodies early on in the tube frame era. I'd leave engines open, but they have to be stock block based. If you want to run a push rod, so be it, but you have to actually have a production version and use the same block. Same for over head cams. We ran an overhead cam Toyota in Nascar, but they had a lot of trouble teching it, so new tech inspectors would be a must in the revised Nascar world.
For production racing classes, I would ban all tube frame cars from the GT ranks. If you can't run with a 911 or M3, stay home. I don't car how much your car looks like an RX8 or Camaro, it didn't roll of an assembly line and it's not as compromised with chassis setup. Make the effort or go home.
This is for any top level series, Indycar in particular, get rid of spec anything. No spec chassis, no spec engine, no spec tires, no spec, no spec, no spec. Create a set of rules and let anyone that wants to build a car show up.
And F1, no green crap. Racing isn't green so quit trying. If you want to run electric cars, create a new series and have at it.
I'm sick of having to go green everywhere...I mean oil is an eviornmental substance. It comes out of the earth unlike nuclear energy or electricity. What could be more natural!? Until they can create a race car that runs on water, leave them alone!
OK, I feel better now!