The one I’ve had in my head for a while is a “Factory” simulator. Like, think Factorio or Satisfactory, but grounded in reality, instead of on an alien planet. You own a factory and take contracts to produce stuff, and have employees that run everything. Occasionally, you’d actually need to tear down and re-tool chunks of your factory to accommodate new production. Initially, you contract-out raw materials, but maybe, eventually, you source and process them yourself.