One of the things that fascinates me most about Undertale is the basically unanimous formation of an incredibly large amount of AUs by the fandom. So many alternate realities where everyone seems to agree on what's canon and what isn't. So much imagination. You can find just about any genre in the vast world of Undertale AUs alone, it's so awesome that people have been so inspired by this little indie game (and each other) that they've woven such intricate, varied stories, to the point where it hardly even resembles the beginning product. They took this game and made it their own. Blows me away!
