Rimworld, way too much Rimworld. A big part of it is that I'll often have it one a second computer during the workday just on pause to pop in during downtime but I guess that still counts as playtime just being loaded.
It is a dedicated and organized team of modders that has output a stunning amount of content, all of it cohesive, and what many would consider required "base game" content.
They have literally put out dozens of mods that could be standalone DLC, all for free.
For me a lot of it is in trying to maximize efficiencies. Setting up task duties and resources where you have plenty enough but not overloading your storage. Defensive position designs to counter any possible situation. Layouts to the base to allow free movement.
Lot of others try for specific artistic ideas or scenario goals, but I'm something of engineer minded player.