*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.
Undertale is a linear story-driven game isn't it? People don't usually mod games that aren't a sandbox in some way.