Awesome! This was a learning experience for me too. Im actually working on other parts of this so it sounds a lot better and flows easier. It helps me learn composition for band too.
Okay, let me elaborate my reasonings. First, Toby Fox has the authority, not you.
Second, FanART is different than FanGAMES.
Third. It is up to Toby Fox to create Undertale for the 3DS.
Fourth. W. D. Gaster is even licensed by Toby Fox, even though you had to hack the game to find him.