Have an event list like in Forza 4. This allows full customisation of player progression.
Theoretically, Forza Motorsport 4’s career mode had up to 310 championships (31x10 grid) consisting of 1,260 races overall, including the Porsche expansion career mode events, each designed to allow any and every of the 659 cars in the game to compete in. Any hardcore perfectionist would be crazy enough to finish and win all of that.