What I think would be the best approach to adding cars.
People have kept asking for it, Sims 2 added it in an early expansion, Sims 3 had it in the base game. Even Sims 1 had the carpools which where a pretty fun indication of career advancement. Especially with no plans of a new sims game, it feels like it's not a discussion any more of IF they will put in, but HOW.
My gut feeling is to make it a base game feature, with like 3 cars in the base game at least, available to all. The boring standard options: The cheap car, the average car and an expensive sports car. Along with a parking spot to put them in at your lot. Just the bare minimum.
I would also strongly advice to have cars be more like Sims 2 where you can see the sims pull up or out of the drive way with animations, and drive off on the road until disappearing when reaching the end of the region.
At the same time cars are added to the base game, a gamepack focused on cars and motorcycles could be added as well. The game pack could add a neighbourhood with a garage, and would add some basic car maintenance stuff to the game along with car modifications that could also be done at the garage. But sims could also do it at home themselves if they are handy and buy the required items for their own garage. They could even make that their business. The new neighbourhood could also have a shop for motorcycles and one for cars, and with the now upcoming businesses and hobbies pack, your sims could start such a shop themselves too!
This game pack could add a variety of cars and bikes that you can modify, and also a couple of cars and for the first time ever, an old bike you can fix up. These you can't buy ready made, sims have to put the work in to drive them, like sims 2 and 3 had a car like that.
And outside of this, some expansions and game packs already released could get a car, bike or equivalent transportation added to them if it makes sense. Like country living adding a pick up truck, city living adding a nice sports car, seasons adding a nice convertible or two of which you can put the roof up or down (keeping it down could be trouble depending on weather!) Journey to Batuu could add Luke's speeder, get famous could add the option to order a limo to pick up your sim, island living could add a jeep, Eco living some solar powered cars, etc.
Future expansions could also add a car or bike that fits the theme.
