Realistic traffic. If SimCity4 could do it, why not Skylines? The whole traffic system in that game is complete garbage and hurts the simulation and feeling of making a realistic city.
With SimCity 4, the traffic you saw was just a visual representation of the game simulating the proportional amount of traffic that was viable to the situation, you would notice this often with cars fading out of existance. However with SimCity and Cities: Skylines, each car is holding citizens that have actual destinations, so unfortunately I don't think realistic traffic has been created in a game yet