I'm leaning heavily on simulation, economics, towns with real economies, and interweaving progression systems. It's a custom engine. I finally have the foundation built, it's multiplayer ready, and it currently loads in under 200MB. The idea is to be hyper efficient to simulate multiple towns that grow by themselves and you can trade and interact with.

https://www.youtube.com/watch?v=BeZ3O6F5FXU

It's a free-time project, but I will happily take investment and make it my full-time project. :) I have a game design-doc that I have built out, and I personally like it a lot. I believe in it's potential.