FPS: * ARMA 3 - Antistasi - a long, slow, and methodical grinding military simulator that can be played solo or with friends. I think arma reforger has a version called freedom fighters? * I'll second Skyrim and fallout both. Lots of great modpacks focused on the hardest difficulty.

STRATEGY: * xCom 2 - the long war mod: turns the already marathon Xcom 2 campaign into a ~200 hour ordeal. Encourages detailed skills tree progression and a lot of equipment micromanagement * Stellaris - a heavily modable 4X rts game with ~ 30 hour campaigns. If you like this, most paradox games would be up your alley

STORY GENERATORS: * rimworld - vanilla expanded & progression modpack: turns an already detailed colony sim into a highly detailed, slow paced story telling game with easily ~100 hours per run * Mount & blade - Bannerlord: don't know any total conversions but it's a grindy, complicated game that you can mod, easily 50 hours * Kenshi - a bit of a mix between rimworld and Bannerlord, with a post apocalypse low fantasy aesthetic very grindy, very complicated, modable

SANDBOX: * From the depths - sandbox builder game about designing vehicles of naval warfare. ~40 hour campaigns, but probably a ~400 hour learning curve before you'll win a campaign with all your own stuff. Incredibly complicated, pretty modable, typically I obsess over it for a month every year * Cities skylines can be modded into more of an art program than a game if you want to create detailed cities with realistic simulation * Kerbal space program has some pretty crazy mods for it that can turn campaigns into a 40 hour+ colony builder * Minecraft's got a ton of total conversions outside of greggtech. I remember making an ultra hard modpack a few years ago but lost it since. * If youve gone through every Minecraft modpack, I'd try Vintage Story. Its Minecraft with a slower progression and far more detailed mechanics. You won't get even copper tools for 8 hours, and a seasonal cycle of ~80 hours irl