i had an idea earlier:
deltarune.com, for a few years before the game's release, only contained him.png, an image with wingdings reading "THREE HEROES APPEARED TO BANISH THE ANGEL S HEAVEN"
so what if, in the same way that people theorize new deltarune chapters now, someone found deltarune.com before the game came out, and theorized about a direct undertale sequel based on the limited info they had?
Arha - A human who lives near Newer Home, a predominantly monster-populated town which was the first settlement created by monsters after they reached the surface. They were born in the area before the barrier was destroyed, and grew up watching its development. Their design is based on the unused human sprite in Undertale's files.
Suzy: Monster Kid's mean sister. Like her sibling, she's a bit of a rebel, and often sneaks out of the house. She is based on the old theory that the "Suzy" mentioned by the Clam Girl NPC is Monster Kid's sister.
Napstablook: You know them already. When they came to the surface, they realized that most humans get spooked by seeing ghosts, so they asked their cousin where she got her body. They were referred to Alphys, who found them a doll to inhabit. It's a friend of Mew Mew that she said Napstablook "reminds her of".
the story basically consists of the three heroes traveling to mt ebott and ascending it so they can reach the angel's heaven. its progression is much more similar to undertale's; no dark worlds, just traveling from area to area
i doubt i'll turn this into a fully fledged story or anything, it's just a fun idea :)