I like all the ideas, especially the tuning unrestricted. Many times I've taken a car out and had just that slight amount of under or oversteer that made it unsuitable for racing, for me anyway, and I wished I could just open up the tuning menu and make some small adjustment to iron out it's shortcomings. And as you said, if we're all running the same lap times what does it matter if it's tuned or not?
For the vintage/modern separation, I'd go so far as label some rounds as "open" and allow any car, any week. Or fix them to "modern only" at the higher PP levels.
I'm as guilty of this as anyone. I have a hard time getting motivated running around a track on my own for more than a few minutes. Even running the TT's, I rarely put more than a half or 3/4 of an hour into it, I just get tired of running the same car over and over on the same track. Once the green flag drops I have a laser focus, the adrenaline is flowing and my lap times begin to fall as I concentrate on hitting every apex and being as smooth as possible, especially when battling with someone.