“Skyrim is a game played on Nexus Mods, in which you try to assemble and run an optimal combination of mods.”
Also Assetto Corsa is all about mods, even the game's menu is replaced with a third-party launcher. I mostly drive the same few cars, but I have about 150 GB of tracks. Most impressively, people added stuff that wasn't in the engine originally: changing time of day, including night, and weather like rain. And it was done by one guy, with further improvements by others.