Perhaps they could have a mixture of "Free Lance" races where any car and mods are accepted (i.e. Sunday Cup, Clubman Cup) while also having some races with imposed restrictions on different categories such as type of car (Spider & Roadster, Sedan, Compact Car, etc...), horsepower, power-to-weight ratio, etc... I also like the aforementioned idea of having races with a restriction on tire compound (just imagine a race where the lineup includes Shelby Cobras on Sim tires!).
Bring back the Event Generator from GT2 where you entered a car and the game would place you on a random track with random cars designed to match your car's performance. They could expand upon this greatly by allowing us to make our own events where we are in control of every aspect of the race, including the amount of AI cars (anywhere from 1 to 5, or 6 if you want to just watch the AI race), the actual cars, level of modification, number of laps or hours, right down to the actual parts and settings of the cars. Wanna make a race where you run a field of Skylines and set half of them up to have great handling and good speed while the other half are set up to have great speed but mediocre handling? Be my guest. PD could get really creative with this. The possibilities are endless.
Another welcome addition would be to bring back the screen from GT1 where you could view the horsepower and weight of the cars you were racing against before the race. Perhaps when previewing a race and looking at a replay of the cars racing, they should have a button you can press to view this type of info on whichever car the camera is on.This would allow people to get a good idea of how fast the competition is instead of having to repeatedly enter and exit the race trying out different combinations of mods and cars to get at an equal level with the competition.