One interesting plot idea could be that Sans uses his mind - reading ability to prevent a major disaster in the Underground. He senses the intentions of a powerful and malevolent force before it can act and is able to rally the other characters to stop it. Another idea is that he reads the mind of a character who is secretly an ally but has been acting strange, and through this he discovers a hidden plan to save the Underground that the character was too afraid to share. Or perhaps he reads the mind of a human who has entered the Underground and discovers their true motives, which are not as bad as everyone initially thought, leading to a different kind of relationship between the human and the monsters.
Well, a plot idea could be that Sans reads the mind of Frisk. He discovers that Frisk is constantly conflicted between different choices in the game, like the choices between mercy and killing. This internal struggle could add a new layer of depth to their relationship. Also, Sans could read the mind of Papyrus. Maybe Papyrus has some insecurities that he hides well, and Sans, through reading his mind, tries to help him in a more subtle way. Another plot could involve Sans reading the minds of the other monsters in Snowdin. He could find out that some of them are actually envious of his laid - back attitude while they have to work hard, and this could lead to some comical or heart - warming interactions.
One idea could be that Alice finds herself in the Underground and has to navigate through the different areas using her unique sense of wonder. Maybe she befriends some of the monsters like Toriel and they go on an adventure together to find a way out for her.
With Papyrus, Sans might become more protective. If he reads Papyrus's mind and discovers his dreams and insecurities, he could be more involved in helping Papyrus achieve his goals. For example, if Papyrus is worried about not being a good enough Royal Guard, Sans could give him more targeted advice. With Frisk, Sans would have a deeper understanding. He could anticipate their actions better and might be more forgiving if he knows their true intentions. If Frisk is on a path of destruction, Sans could try to sway them by appealing to their better nature that he reads in their mind.
A crossover with 'Sherlock' would be cool. Imagine Sherlock's deductive reasoning combined with the BAU's psychological profiling. They could be working on a case that involves a criminal mastermind who is playing mind games with both teams. Sherlock might initially be dismissive of the BAU's methods but then come to respect them as the case progresses.
Another plot idea is a story where the antagonists from both games, such as Flowey from Undertale and the King from Deltarune, form an alliance. Their goal could be to take over both universes. This would force the heroes of both games to work together. The fanfic could explore how the different characters with their own powers and personalities interact and strategize to defeat this powerful alliance.
One idea could be that the characters from Homestuck end up in the Underground of Undertale. They might have to deal with the various monsters there while also trying to find a way back home. Maybe John Egbert could team up with Frisk and they use their unique abilities to overcome challenges like puzzles and battles.
One idea could be that the animatronics from FNAF end up in the Undertale universe. They might be mistaken for strange monsters at first. The Sans character could be particularly intrigued by their mechanical nature and try to figure out how they work.
One idea could be a bitty sans getting lost in a big, strange world outside of his usual tiny environment. He has to find his way back home while facing various challenges like avoiding being stepped on by giants and finding food sources. Another plot could be about a bitty sans forming an unlikely friendship with a non - bitty character, perhaps a human who at first is scared of him but then realizes his cute and friendly nature.
One interesting Sans Undertale fanfic is 'Inverted Fate'. It offers a unique take on the Undertale universe, with well - developed characters and an engaging plot that keeps you hooked from start to finish. Another great one could be 'Swapfell Sans - The Cold - Hearted Protector' which delves into the character of Swapfell Sans in a really interesting way, showing his complex personality.
One idea could be exploring an alternate timeline where Sans and Papyrus have different roles. For example, Papyrus could be the more serious and powerful one while Sans is more carefree. Another idea is to write a fanfic centered around a new character who gets trapped in the Underground and has to navigate through the various regions, making unique friendships along the way. And a third idea could be a fanfic that focuses on the backstory of the monsters before they were trapped underground, showing how their society was and how it changed over time.
The idea of Sans being able to read minds in fanfics is interesting because it adds a whole new layer to his character. He already has this mysterious aura in the game, and mind - reading takes it to a new level.