http://unknown-horizons.org/
https://github.com/unknown-horizons/unknown-horizons
After longer hiatus the just release a new version that is now Python3 compatible, see: http://unknown-horizons.org/blog/second-post/
They also mention to possibly switch to the Godot Engine:
We are searching Devs for the Godot Engine. We want to switch to Godot because the fifengine and the gui lin fifechan is not really active in development. And have bugs. If you are interested on help us to switch to Godot join your discord server.