Game Idea: Competitive Multiplayer Economy / City Builder - test out your nerdy economics ideas against those of your friends'.
Crazy shower thought after a debate with a friend lol, but I thought that version of Sim City that came out a few years back was kind of cool for being able to make a city in the same region next to your friends, I think you could trade and stuff and also contribute to regional public works etc. (never played cities skylines so IDK maybe that wasn't unique to Sim City).
This would need to be a little more "systems-based" I think, because the idea is to set up various rulesets (or evolve into them as your little tent village grows - basically locked talents that you select), such as capitalist, communist, imperialist, Amish (lol!) etc. there are either some win conditions set ahead of time based on different game types (to see which type of civilization is best at a taking things over, generating happiness, etc.), or maybe custom ones.
Super low detail here but the sky's the limit, I really think it would make for a killer game night game that you can play for more than just weekend friend slop if done well enough.