Hmmm. I like this question.
I would start with the basics of what we have seen GT5 will be/might have inlcuded. All the tracks, cars, possible weather, day/night..... I would have tracks and cars from pre WWII and on. I would make it playable as is now, and has been.
Then I would add a role play option. Pick a date to be born. From then on you cultivate your racing career. It turns out, as it turns out. No redoing races. You crash, you crash. You lose, you lose. Start at a young age, and try to work your way through the ranks. And keep it time specific. As today, if you wanted to get into F1, you start in karts. Yes, karts. In the 50's, people started later, and in typical race cars. No matter when you start, as time goes along you are able to watch as the cars actually progressed. Crossing the decades and moving into the newer cars as they came out. Imagine starting in the mid 60's, and working your way up and through the 70's. Moving from early Trans Am, to Le Mans cars of the 70's.
Sorry if this rambles, but I think this would be a lot of fun, and have virtually endless possibilities.