Tilemap Town
A long time ago, BYOND was pretty popular and a pretty common genre was the "building game" which was probably the easiest thing to make. It was mostly just a chat that had a map attached to it that players could freely build on.
The best game in this genre was Icon Ultima and it brought me lots of years of enjoyment before it finally died. This is pretty much a recreation of that game as an HTML5 web application, and it's Free Software. It will expand on the idea of a building game and also focus on using the world that was collaboratively constructed for roleplay.
Python server
Just run runserver.py with your Python 3 interpreter, after installing the websockets library.