Here's what I would do:
GT5: Pretty much like GT3. Simulation mode with cars to win and parts to buy. More specific-car restrictions, like actually having to use a British car in the British GT. License tests and time trials, OK. Arcade mode but not required for completion. A good selection of road and street courses, both real and fictional. Delete rally. The main GT5 disc would include ALL the cars from all versions (see below).
The main disc would be $50-$60. Then there would be $20-30 expansion packs:
GT5R: Real rally. Real and fictional stages, but organized the way a real rally is. One-way stages, with starts staggered by 1 minute between you and the AI, with time on the last stage determining order on the next stage. That way sometimes you'd be on a fresh stage, sometimes you'd be in the dust cloud, and if the AI in front of you wiped out badly, you might pass him.
GT5X: Autocross. Street and prepped cars, with classes determined by mods you've done. A couple of different backgrounds; a thousand cones, a sea of concrete, and a lake of asphalt. One-car-at-a-time runs for Solo II, or heads up against an AI car on a mirrored course for Pro Solo. Lots of repeatable events to practice specific course layouts, but also with a random course generator (after all, it's only cones your moving) and a course editor so you could put together and save your own course layouts.
GT5D: Drag. Heads-up and bracket racing, with classes again determined by mods. A couple different backgrounds.
GT5D1: Drift. Drift to your heart's content. Real and fictional road and street courses, touge and canyon runs, etc. etc. etc. Style points and/or timed runs.
This way nobody would have to do the parts they didn't like just to get 100%, and PD could really concentrate on doing a good job on the individual pieces instead of trying to do all things in one huge game, and doing a mediocre job on some of them.