I describe Cities:Skylines as a "city painter". You can make nice looking things in it, but there's little challenge and most of the mechanics don't have a lot of depth.

I'd suggest looking at SimCity 4 for the peak of the in-depth city simulator. The systems in it go way deeper, it isn't limited by inevitable scaling problems with agent simulations (Tropico, even C:S), and there's a large, long-running modding community for it.

There's a few mods that would be considered basically essential, namely the Network Addon Mod (which overhauls the traffic simulator, in addition to a vast amount of content).