I've been working on https://semicity.derg.nz for the past 1.5 month ish and it's turned into a rather advanced sim game so far.

It has:
- a region (map) generator with height, hydraulic erosion and river meandering etc
- region-wide road/train freight simulation
- resource mining/growing/etc
- disasters, fire dept, etc
- parks and happiness
- full public transport (can make walkable car-free cities!)
- multiplayer (resource pooling)
- so much more