Map editors and modding have always been pretty common for both turn-based and realtime strategy games, and the entire MOBA genre originated from RTS mods. Beth

Map editors and modding have always been pretty common for both turn-based and realtime strategy games, and the entire MOBA genre originated from RTS mods. Bethesda RPGs have active modding communities in part because they always need community-made patches to be playable. Doom and Quake and Unreal had very fruitful mod and fork ecosystems with offspring that went mainstream like Team Fortress and Counter-Strike. Several simulation games shipped with the Gmax 3D modeling program.