X3: Terran Conflict - Ignoring the story plot and just playing sandbox ("custom game") makes you nothing more than a citizen in a universe that doesn't care if you succeed and success is a long, hard road to the top of whatever avenue you pursue. One of the best space games there is. It's also moddable and there are some awesome mods out there to make the game even better.

Mount & Blade: Bannerlord - Basically the same as above; but in a medieval wargame/RPG... It's pretty unique in gameplay so I'm not sure what to really call it. You start off as just a dude and can work your way up to becoming a king and conquering the entire country. The combat is part large-scale strategy, part 4X and part action sim as you move units around a world map for positioning and getting to cities and outposts, and battles put you in control of your singular dude swinging your weapon with some nice mouse controls, while also able to command your literal thousands of men in moment to moment tactical decisions.

Dwarf Fortress - Specifically Adventure mode in the pre-Steam version (since Adventure mode is not yet in the Steam version). You're literally whatever character you create living in a fantasy world. It's a simulation more than any other kind of genre. You can basically do whatever you can think of. You can be whatever you want up to and including a literal god if you work hard enough. There is no story other than the history of the world, which the game records and even after you die and make a new character in the same world, any mark that previous character left continues to exist and can affect others. Like say you steal from a guy, get into a fight and end up killing him. His son might seek revenge on your character, succeed and then fall into a depression. It's kind of a mad lib of sorts so you have to have some imagination, but it's the most complex game that exists right now.

Kenshi - Nobody likes you, you own nothing, you're weak as shit, the world is massive, and you can also basically do anything if you work at it. Command huge armies. Become a robot and forego the need to eat. Build cities. Conquer the planet. Not only is the game play incredibly fun and rewarding, it has a super interesting world with plenty of cool lore. Plays kind of like an ARTS and has complexity that comes close to Dwarf Fortress's.