Well, since you asked.
Let's add stochastic elements to this game. That would make this game much more realistic in many ways, so:
1. Varying pit times, as determined by an algorithm that takes into account the appropriate probability distributions that describe the inherent variation in times of all the processes involved in a pit stop. Even in the real world, accidents happen, flukes happen, and pit time changes between stops, even by the same crew in the same race. Sometimes, your pit times will be shorter, sometimes it'll be longer. Good luck.
2. Part and vehicle failures based on varying reliability between parts. Have MTBF and the failure distributions available before purchase of a part to help user determine which part better fits the user's needs.
3. Possibility that GT Auto and whatever places fixes vehicle damage is busy with other customers, so you must make an appointment and will have to wait to get your oil changed or damages fixed. The number of people in front of you is determined based on principals of queuing theory.
In addition:
1. Weather forecasts available with a certain degree of accuracy that changes. Weather may or may not be actually be as predicted. Also, weather depending on the season and location, reflecting real life climates.
2. Ability to buy raw material where prices fluctuate based on actual and anticipated supply and demand constraints, and access to a full machine shop to manufacture and post-process your own parts based on the purchased raw material. Have the option to manually choose feed rates, spindle speeds, heat treatment ramp-up and ramp-down rates, etc. The raw material will cost money; the manufacturing will cost money (in terms of credits per hour). If one screws up the manufacturing of a part, then the material becomes scrap and new material must be purchased. Also include machine break downs.
3. Full industry-level CAD, FEA and CFD tools to design and analyze own parts with correct material specifications. Incorrectly designed parts will lead to failure during use.
4. Failure analysis: ability to analyze failed parts to understand what went wrong and what can change and be improved in the future.
5. Finite number of cars, with a specific "production rate" to increase the number of cars available over time (much like in the real world). Out of production cars are out of production. Number of cars available will not increase. Scrapped cars count against the total number of cars.
6. Creation of a Gran Turismo market where used parts, vehicles and privately held memorabilia can be bought and sold. Price of various items to be set by seller, with negotiations between buyer and seller allowed.
7. Changing price of parts based on market supply and demand.
8. Depreciation and appreciation of all parts, vehicles and related memorabilia as determined by the greater Gran Turismo market.
9. Ability to borrow credits in game with either a fixed or variable APR. Borrowed credits must be paid back with interest.
10. Ability to default on loans, which will result in a PSN account ban.
11. Taxes must be paid on credits earned. Taxes will reflect your local rate.
12. Random political and civil unrest that causes cancellation of races.
13. Ability to mount gun racks on my pick-up trucks and dangling testicles on my vehicles.
14. B-Spec Bob have the ability to do or be subject to anything that a person may do. B-Spec Bob could engage in drinking and driving, crash and die, on his own free time. Likewise, B-Spec Bob could be a victim of road rage.