Right now, every citycar can spawn in every city anywhere on the map, as long as it is the right time. Which is a bit boring.
What if citycars are spawned by buildings? Say, every building can define one citycar it spawns (on the nearby road), and a spawnrate (which is still modified by the general settings). This would mean:
a) different climates would have different citycars
b) since spawn rates can vary, you could have neighbourhoods with many cars and those with less cars.
c) you could bind special vehicles to special buildings. For example, you can have a fire station which spawns firefighter trucks. Or, if every building gets the parameter, you could have a taxi-station-extension which spawns taxis, or even a factory which spawns more vehicles the more active it is, eg. a furniture seller could spawn trucks to deliver their goods to the homes of the consuments.
This does not really add to the gameplay and is mainly eyecandy. But then again, that's what citycars are anyway.
From what I have read, city cars cause major performance problems.
Also if you are playing well there should be no city cars as people are ecologically friendly and use public transport if you have >90% service rate.
City cars with destinations cause performance problems. Increasing the number of city cars in total will also require more processing power, but this is a user-tunable setting already. A little more logic into which city cars spawn shouldn't change much. I suspect there is already some interaction between building levels and city car spawning. Stations also spawn pedestrians when trains arrive, or so it seems to me. However, having lots of taxis appear arround busy bus terminals might be more of an annoyance, than eye-candy.