Some of my ideas:
Needs and bonuses
Different car types offer different types of functionality. Small, cheap cars can fill your fun need but have a chance of giving an uncomfortable moodlet depending on their condition, more expensive cars or cars with good conditions will fill your fun need and can give you a happy moodlet. You can also fill your bladder or hunger need by choosing to visit a rest stop and if you travel with sims you can replenish your social need. The number of seats determines how many sims that you can travel with in the car.
Conditions
All cars have conditions, you can buy cheaper cars that have worse conditions (used cars), and you can fix them by using a new skill called mechanic or pay to take them to a car mechanic. If you drive a car that has too low condition you can get busted by the police or the car can break down. Upgrading cars can also be a way for sims to save money, because they can buy cheaper cars and keep them in good condition. They can also make money by buying cars in bad conditions, fix them and then sell them for money. You can also buy used bikes and boats and fix them and then sell them with the mechanic skill.
Upgrades
You can upgrade any car to give bonuses, like for example upgrading your car's stereo will give them a "listening to good tunes" moodlet. You can also choose to upgrade your car to be faster or to be more comfortable. You can sell upgraded cars for a better price.
Fuel
All cars require fuel, some cars have better fuel consumption than others (petrol, diesel, hybrid, electric), you can refuel your car by clicking on it and then you will drive away to a rabbit hole. Electric cars take much longer time to refuel and are cheaper, but needs to be charged more often unless you charge them at home, but that will increase your electric bills. You can buy chargers to decrease the time to charge them at home.
Racing
You can choose to go racing with any car, they will work like the space rocket. Where you have a rabbit hole with a higher chance of succeeding if you have a fast car in a good condition and with upgrades. With different races available that are more and more difficult. You don't have to own a sports car, you can also race easier races with a cheaper car with a lot of speed upgrades. You will get money for going racing but you will degrade your car's condition more by doing it. You can race legally, or you can race illegally with higher risks and higher rewards but you risk getting busted by the police.
Customization
All cars can be customized, you can customize colors, wheels, and the interior, racing cars can be customized with spoilers, decals etc.
Parking spots
If you place down parking spots on community lots sims cars will spawn there when sims are visiting the lots. You can look at other sims cars like you are looking at a decorative item.
