*edit. I should note a good modding api and/or editor is great for any game.
I started programming though making mods for battle for middle earth 2.
However, we must make sure to keep in mind the ways in which capitalists will exploit the goodwill of developers and free software wherever it may be.
Every Bethesda game has 3 essential mods: the mod that fixes all the bugs, the mod that makes it harder, and the mod that let's you skip the intro