I mean you don't have to stick to undertale if you don't want to but maybe Fresh Sans would be cool to add, like you just encounter him ? I dunno. Maybe also horrortale. I think what also might be cool is like a side(?) mystery, you might already have that planned I have no idea but still. Though it might not work with what kind of story you are going for if you it's like just romance or fluff or something.
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I've been talking with my friends so I think I might be able to get help with the main concerns I have about using undertale as the fandom, so I think that per will be staying!
I think Horror can be interesting as I do like him as a character, plus I can see plausible fluff for him for the romance part. Fresh would definitely be added lol.