Personally, I'd be willing to pay more than $60 for GT6 in order to get more content. It's basically the only game I play, and it lasts for years, so I could certainly pay more like $100, just for the game.
That would allow them to hire more staff to do more modeling. I'd love more tracks. Don't care as much about cars, since there's just a ridiculous amount already. If they just add another 300 or so late model premium cars, plus maybe 50 iconic old cars in premium version, I'd be happy with that for cars.
If they added maybe 10 more tracks from what we currently have, that would be perfect for me. Sebring, Road America, Spa, Monaco, Mosport, Isle of Man, Brands Hatch, Pike's Peak, Mid Ohio, and Sears Point. Those plus the current track list and I'd want for nothing.