The Cape Ring is an excellent addition to Gran Turismo but the track I’d most like to see (and the one I’d faithfully recreate if I were a gazillionaire) is Grand Valley Speedway. Fast, flowing, lots of places to overtake, very difficult sections and wonderful scenery.
My favourite track on the game is Deep Forest, but having an F1 race there would just be ridiculous. Trial Mountain is the same, both are fast, bumpy and narrow and would be far too dangerous.