That's exactly how I have been thinking. What I was thinking was to make it a Steam game, but entirely open ended, maybe with a bit of resource scarcity so that people have to scramble to get what they want. I said to someone else yesterday maybe things like cities with user built transportation systems for rent, resource wars with medieval siege engines all designed in an arms race between users could evolve organically. In a way that combines the two things you mentioned: the shared aspect and the incentive to learn programming. When I was a kid I knew all the pokemon and their stats, why? Because it was required to advance in a game. Now suppose you have a castle and someone figured out a way to outsmart your defenses. Either use programming ideas to update your cannons or lose the castle... I could add a basic "building block" of damage to avatars like being hit with anything moving faster than x, then anything you can design that throws things could be a weapon, figure a way to store balls "under pressure" triggered by a step plate and you have a bomb/booby trap.

edit: (sorry I keep having ideas and editing the post) At some point the game itself might be open kinda like the internet, the reality of the game what things exist where, who has what can be agreed upon with some sort of blockchain system, then several "browsers" can be written to "plugin to the matrix" if you know what I mean.