collaborative healthy open source mobile multiplayer fantasy boardgame

I dream of a simple, moddable, open-source fantasy board game engine where it is easy to participate the creation process at different levels of creativity. The interaction in the game should be very simple, while the content diversity could be immense. The game can be played in multiplayer mode over the course of several weeks but with very little screen time required. It should be free and without ads, serving as a playground for people who want to explore game development creatively without having to create their own game from scratch.