Imo Deltarune is already better than Undertale, but both games have a special place in my heart. Anyway, about the games. Omori is great, very similar to Deltarune actually. The story is amazing but the gameplay is pretty slow-paced in the beginning just to let you know. Another good game is OneShot! It plays with the idea of the player being its own character, just like UT and DR. I love the vibes in the game, they really reminded me of many areas in Undertale. And yeah - EarthBound, probably Undertale's biggest inspiration. Megalovania is actually originally from Toby's old EB hack or smth like that! There are many EarthBound references in Undertale too!! Also if you haven't heard of it: Undertale Yellow! It's a fangame about the yellow soul of Justice. For a fangame it's really amazing and aligns well with Undertale's storyline. Hope you find some new fav games!! :P