@Imaginativ3
I love, LOVE your idea. It's so comprehensive and well thought out.
The only thing that I would change is the part about city-dwellers not owning cars. My main Sims are in San Myshuno. In my story, the brother rides a motorcycle. But his sister would definitely own a car. She's not subway material whatsoever. :D
Anyway, I was thinking that, since all buildings in San Myshuno have elevators, the elevator could have new options: Go to the Lobby or Go to the Garage. From there, they'd be taken down to parking area that is its own lot. Either it can be a basement garage, as lots of apartment buildings have. Or, it could be an above ground parking structure.
Car owners can run into other tenants there or other passers-by. Each tenant would have their designated parking spot there they could do car repairs, wash their cars, etc. The best part: Sims can get in their cars, pull out of their spot and drive off the lot. If it's a below ground lot, they'd drive up a ramp and then teleport. If it's above ground, they could drive to the roll up gate, press a button to open it, then roll onto the street/teleport.
The garages could be places similar to the hospital or the police station -- Sims would only go there to retrieve their cars. There could be a different parking lot for each neighborhood in San Myshuno, for the sake of immersion and variety.
In any case, I truly hope that TPTB are reading some of the feedback from these forums because your idea has been one of the better ones on a message board that is already overflowing with great ideas. :)