I think if we got functional cars it would just be taxis, ubers, and school buses that just appear at the curb in front of the house and drive away like the cars and trolleys you can already see on the streets in the game.
They could also give us stationary cars/motorcycles that your sims cannot drive, but can work on, upgrade, give a new paint job, show-off, cry in, listen to music in, ect.
As far as transportation objects go, I'm personally hoping for bikes, skateboards, and scooters for all ages to earn fitness/movement skill, and to exit a neighborhood (like instead of walking to the edge of a neighborhood and hitting a load screen, you could bike there). This would also be good for places like San Myshuno, which doesn't have roads in playable areas. You could just keep them in inventory or a bike rack object, and the neighborhood terrain doesn't have to be changed, even if you don't want sims to just appear on the road or sidewalk on their bike/skateboard/ect.