I've been thinking about having just one fuel tank for both gasoline and diesel, so you don't have to swap them every time, and you can lock up your engine if you refuel with the wrong type of fuel. Additionally, implementing water consumption for radiators would mean that if your water is low, it will decrease the efficiency of the radiator. Also, a significant addition to career mode could be oil consumption and different types of oil, requiring you to buy the right one for your car and monitor your oil level. Could also use some of the dash lights.
