While I don't have the time, skill and determination, one idea for a game that I really like is to do a Dwarf Fottress like autism project that would basically be Frostpunk but far far more complex and in depth.
To make a twist on the Frostpunk idea it would be set in the 1920-1930, and instead of a steampunk aesthetic have a sort of brutal Stalinist era 5-year plan new industrial cities feel. Also you'd have different faction who would be the ones making the refuge sites. Soviets, Brits, French, Americans and maybe Weimar Germany, all starting with differing societal laws, technologies and weaknesses, perhaps even completely different plans for survival. The tech would not stay stagnant, instead going from irl stuff to things like transistor computers and atomic power. Instead of graphics it would probably have visual backgrounds, while main gameplay would be through speeadsheets, perhaps similar to CitK, but not as obscure as the space Aurora4x game. The whole complexity would come from a need to plan the entire economy, including basic stuff like clothes, tools and so on, which are abstracted away in Frostpunk. In many ways the whole charm at least for me would be that the game would serve as a microscopic version of in-kind economic planning, as you would have to maintain your refuge of a thousand or so people. Hell, you could make later down techs basically be advances in automated planning, which would let the player be replaced by an AI on more and more tasks, letting one to properlly manage more, thus expand more. Stop just scrapping by and surviving and start thriving, establish contact with other refuge points, reestablish society, fuck it, end game goal - build a rocket and orbit the Earth, then develop special chemicals allowing you to slowly thaw the Earth.