There are few sections where you can ride as passenger in NPC car and I always used that option when it was available. I enjoyed the city but there's so much of everything that I appreciated the opportunity for in-game down time too, to absorb everything.
What is missing in a car system is having people who actually park their cars at parking spots, get in and out etc. All of them are just driving for hours.
There's RedCab and Battle Cabb in game. How about if taxis picked up and left passengers off in certain spots?
Empty parking space is what I appreciate in City Center and Watson. I was also thinking if NPC cars would enter and exit from those big parking garages, but there's a problem with what to do with NPC character then, if player followed those cars.
Edit: Perhaps have NPC cars letting passengers out from car near night clubs at certain times.
Problems I see in general are:
If NPC cars can navigate back into traffic (look for opening or NPC cars give them way, well it's Night City, nobody would)
NPC cars to reaction to player vehicle, in case V uses that spot, what NPC cars should and could do? Have them circling around the block?