This combined with the “fractal game” concept.

It would be a big open-source project (I mean it could be closed-source but it’s a massive effort with massive risk, it would take a billion-dollar company and I doubt any wants to invest in this).

It starts out as just a super-general world simulation, but people can flesh out the details by providing more specific simulations and also mini-games. All of the mini-games are optional, and the localized simulations aren’t run if they’re not requested because they get blurred out* into the bigger simulations (e.g. you can simulate population and income without caring about one cities’ paper production efficiency), so contributors have a lot of freedom in what they can make.

e.g. someone creates a “Fifa” style soccer mini-game. If you don’t like playing soccer, there is also a basic “soccer-management” style simulation where the soccer teams play against each other automatically and the team rankings / income / effect on news and culture will update. If you don’t even care about soccer, the soccer/management simulations won’t even load, and the effects on economy and culture will be blurred out.

How it could be implemented: there is a massive shared database of resources on everything (population resources, income resources, hospital resources, building layouts - all by location), and functions to automatically compute these resources over time when the players not explicitly interacting with them (e.g. update population and income, but also generate new cities and building layouts). Basically, everything in the game has data by some sort of location, an automatic state transition function, which may take other kinds of data and other locations, and possibly a way to manually interact via a user-controlled simulation and/or “hands-on” interaction mini game.

Along with this there is a standard-issue game engine and libraries which the smaller simulations and mini-games are built with. Each of these smaller-systems and mini-games are a module which can be loaded in when they are requested, but are “blurred out” by default. Initially only the global simulations are enabled.

The player makes up their goal: it may be to maximize the worlds income and happiness however they want (e.g. by building nice buildings, an efficient factory, train stations). Or maybe the player is evil and wants to kill off the population via bad decisions which cause the economy to crash, and unsafe research causes a deadly virus to be released. Or the player just wants to build cities and roads which are fun to race in and then drive a race car around everywhere.

Anyways, it’s obviously super ambitious but it would be a nice experiment. Like a generalized, open-source, modular reimagining of Dwarf Fortress.

* When a simulation is “blurred out”, I mean it’s affects are roughly estimated when the user doesn’t explicitly load it. Otherwise a) the game would slow to a crawl because of 10,000 simulations running at once, and b) a poorly-implemented simulation (e.g. which allows the player to generate infinite money, or just crashes a lot) won’t ruin the entire game, the player can just ignore or even specifically disable it. Simulating every minute detail of the world is a kind of hard problem, but since this is ultimately a video game we can just ignore 99+% of it, throw together some basic population and economic theory, and later on transportation theory and culture/politics sim etc., and say "close enough".