some undertale au ideas
- Scienceswap - au where alphys and sans swap personalities
- Dusttale: Tale of the Two Demons - Dusttale but the monsters start seeing sans as they do frisk in the genocide route. Something neither human nor monster. A thing that has crawled out of the deepest depths of hell to vanquish all of Monsterkind. They don't even recognize him as 'sans' anymore. All they see when they look at him - no, it - is a being with only one goal: to destroy the world until there is nothing left.
- Bones Caked With Webs - au where sans and muffet swap. I'd imagine that in this au, papyrus is like 8 years old and sans needs money to take care of him all by himself (single mom sans yippee) so he sets up stalls throughout the entirety of the underground and has his gaster blasters help sell shit. Muffet would probably have found some way to earn the title of royal judge so she doesn't need to do the whole bake sale thing!! she'd also be significantly more chill in this au since she's fucking set in terms of money
- A Striking, Crimson Red - ok so y'know the theory that sans doesn't bleed blood in the geno route, but rather determination? Well.. what if after the slash, sans' "determination" basically takes control of the empty husk of his soulless body? You could go a lot of routes with this one. Maybe "sans" keeps attacking you, as a result of his last remaining will being to make you quit?
- Patience Can Only Get You So Far - Sans somehow manages to get his hands on the SOUL of Patience before fighting the human and, based on a hunch that this has happened before and won't stop happening, absorbs it. Now, he has to deal with a clueless kid who hasn't been alive since the 6th century and attempt to anger the human enough to get them to quit.
- Golden Clovers - On the genocide route, before being killed, Flowey manages to get his hand on one SOUL. The SOUL of his best friend (second to chara, of course!). The SOUL of Justice, burning a bright yellow. After absorbing it, Flowey and Clover have to work together to stop the player.
- Strike Back Against The Hungry - Horrortale Genocide Route. Aliza learns, after many painful, brutal deaths, that instead of being fearful herself, she can inspire fear in others in order to successfully save and load.
(Feel free to use/expand on any of these ideas!)