I'll start with the easy-to-understand and slowly move on to indecipherable nonsense.
1. Maybe the player should be able to own multiple cars and own/rent multiple places to park, with free parking being "just park it on the street here"?
a. Street parking always runs a risk of about 1%/month to 5%/day (depending on where you park) of causing damage to your car, and there's an ever-present risk of part or car theft (depending on where you park). Parking in the worst areas of town with the doors unlocked is a great way to loose cars, whereas parking in a quiet area with power locks and an alarm lets you park almost indefinitely. Improper parking, however, always carries a risk, similar to the risk of having a car stolen, and the consequences range from a $50 ticket to being towed.
b. Parking lots work similarly to street parking, but anything fenced-in or with restricted entry lowers the theft rate significantly (almost to 0%). Fenced-in lots tend to be permit-based, so the player will need to pay for a permit and keep it updated at the risk of being towed.
c. A garage, rented or bought, will ALWAYS have the lowest risk of being broken into, and it will be impossible (or NEARLY impossible) to have the car damaged in any way. Security measures range from a rotten old door with a busted lock to a nice sturdy door with a deadbolt and security system, and the player is allowed to store things in or (in the case of buying garages) modify the garage to their liking. Perhaps housing could be added?
d. There should only be a maximum of five showrooms or extremely fancy garages in the game. These showrooms often have the option of being tended to by mechanics and detailers (who have to be found, hired, and paid) to keep your car(s) in perfect shape and impossibly clean, and almost always include security measures to prevent thieves from getting to your car(s). Be careful about who you hire, though: some thieves are very good at doing whatever it takes to get what they want, and some of your opponents (mostly the well-off ones) are willing to hire people to sabotage your car(s).
2. Wherever a car is parked, it will always be affected by its environment. For example, a car parked on a busy street will collect dirt like nobody's business and slowly have its paint damaged or ruined by the sun, rain, snow, etc; salted roads will also cause very slow body and chassis degradation if the car is not kept clean. Even a car left in a garage will collect dust, if the garage is not kept clean and "sealed" somehow.
3. For every car you buy, you'll have to do paperwork and pay taxes on it. Don't worry, this isn't Papers, Please; the paperwork is filled out automatically, but you have the option of "accidentally" underestimating the car's worth (for tax reasons), at your own risk. The smallest punishment for this is a funny look from someone at the DMV, or being asked to fill it out again with correct/"correct" information. At worst, you'll be cited a month or two down the road.
4. The ingame calendar should include every important date, including the dates of big races, paperwork deadlines, bill deadlines, paydays (for your employees), and anything the player wants to make a note of.