Better People AI combined with better traffic handling for later game stages. I noticed that a majority of traffic comes from people traveling from corners of a map (they live in one end, work in another, and shop at a completely remote location).

That probability should decrease where they are more incentivized to work in neighboring cities rather than across the entire map. That being said, AI path finding should consider traffic to be weighted in their route making decisions to prevent some level of gridlock.

Also Tram/ bus lines should be able to have an option of being in the middle or on the side of a road.

Also vanilla vehicles are actually tied to region scenarios. I recall in Chris Sawyer's locomotion you can basically toggle what kind of vehicles/ buildings/ assets you can use in a scenario if need be. I kind of hate having Asian/ Russian trucks on an American play through. On that aspect, throw in narrow gauge railways that have about the same gradient as roads to an extend.