I'd like to make an MMO factory economy game
EVE Online is one of the only MMOs in existence which has high-quality economic systems, such that free trade is left unimpeded by the game's logic (and the system is well-designed enough that this doesn't break everything). The player-driven economy that emerges from it is fascinating. Read about it if you'd like. I'd like to build a game that focuses almost entirely on this sort of economic gameplay: a game where the primary goal is maximizing production, and people are forced to cooperate as part of a large economic system to achieve it.
The game would be broadly Factorio-like, but on a very large single-shard map. Players could purchase and trade freely definable plots of land, and build up farms, factories, and housing on those plots. These would be built in mostly automation-game-like ways, albeit with significantly more flexibility and freedom for optimization (everything having tradeoffs, of course). In order to keep this interesting, encourage cooperation, and disincentivize copy-pasting, the game's systems would be massive and sprawling. Rather than having a little over a hundred carefully crafted, unique recipes, like in a single-player experience, there would be tens of thousands of unique items, many of which are mostly mechanically similar. The primary aesthetic of the game's design would be maximalism.
Simulate a weather and climate system, to make location choice interesting and introduce variations in regional production. Keep transportation costs high to ensure regional variation in prices. Allow players to combine a vast array of parts to build different vehicles, to introduce new optimization in cost vs. complexity vs. performance vs. efficiency vs. etc. tradeoffs. Each of these systems would not need to be maximally polished: the polish and quality of experience comes from their sheer scale. Each system adds richness to the others by contributing to the massive optimization problem inherent to any economy. Not only would each system add directly to a single player's experience, by pushing them to more thoroughly consider how to design their production, but those considerations would show up in prices, which then shape the optimization of other players. For a very simple example, if iron is plentiful upstream from an important town, that will shape the factories players should build in that town!
Since comprehending (and optimizing for location, weather, etc.) the entire supply chain is a nearly impossible task, players would be pushed to rely on each other for crucial resources, intermediate products, and services (e.g. shipping, market making). The game could also place further constraints on how much a single player could act across the entire supply chain, such as requiring 'skills' to access certain technologies or limiting the total complexity of a player's automated vehicle programs.
The effect would be to produce an exciting and deeply interesting experience of participating in a massive dynamic system, which also functions as the most accurate simulation of an economic system ever produced.