Basically Dwarf Fortress, but sci-fi, with space travel and with really good 3D graphics.
An overly complicated simulation of an entire universe where I can do anything that the game's laws of physics allow me to do. Particularly, I dream of a game where you can fulfil Star Trek fantasies of solving convoluted problems with equally convoluted solutions; but without it being a pre-programmed option. ("Reconfigure the deflector dish to output a neutrino wave to counter act the tachyon field!")
Fake physics that allow for real fake science and engineering.
Close, but RimWorld's mechanics are nowhere near the complexity, even with mods. And I don't just mean with how many different types of rocks and trees exist. I've gotten as close to a DF-like experience + the space travel part, but it's still like 1/4 of the full experience.
Although it's moddable, which boosts it's playability a bit more. I've played more RimWorld than DF at this point.