If you could make any "simulator" game, what would it be?

A rocket design simulator in the same sort of vein as kerbal space program or juno new origins/simplerockets, but as realistic as reasonably possible and with as many options as can reasonably be programmed in (so for example, rather than just placing an engine or getting to specify a couple parameters like nozzle size, you'd have to specify the power cycle and number of combustion chambers per turbopump, size and construction material of various major components, fuel and oxidizer type, etc)