If I was making GT5 I would scrap Nascar, WRC and other special licensed race events. I have nothing against any of those I just feel that it would be better to use generic or tune regular cars to match those standards.
I would also focus on a car creator mode. Literally build cars from the ground up(in a way that works for a console game). Use a system like that found in LittleBigPlanet (just more robust and have mouse and keyboard support) that is easy to use but allows a lot of flexibility. Basically have lots of premade parts(chassis, body panels etc....) that are both unique and from existing cars. These parts can be modified and shaped/trimmed and the system would allow parts to automatically adjust or fit together. Then obviously include the various types of engines, placements, suspensions etc.... but then also account for weight distribution (on all axis) as well as aerodynamics.
This would be my dream GT game. Similar in set up to that game APEX but more varied then just parts from concept cars.