In this fanfiction, Sans could be in an experiment gone wrong. Scientists in the Undertale universe (if there were some) were trying to find a way to bridge the gap between monsters and humans. Sans volunteers or is accidentally caught in the experiment. As he turns into a human, he has an identity crisis. He's used to being a laid - back skeleton, but now he has to face the more complex emotions and physical limitations of a human.
Well, one possible plot could be that Sans discovers a mysterious magic spell or artifact that gradually transforms him into a human. As he changes, he has to deal with new sensations like feeling pain and hunger in a more intense way. He also might struggle with how other characters in the Undertale world view him now that he's different.
It could be another monster who is jealous of Sans getting all this new - found attention as he turns into a human. This monster might try to stop the transformation process or make Sans' life difficult in his new human form.
Maybe in the fanfiction, there's a scientific experiment gone wrong. A human scientist was trying to find a way to bridge the gap between humans and monsters. Sans, being a curious skeleton, volunteered to be a part of the experiment. However, instead of just creating a connection, the experiment transforms Sans into a human. As a human, he experiences new emotions more intensely, like hunger and fatigue, which he didn't really have as a skeleton. He then uses his new human form to try and convince other humans that monsters are not to be feared.
Well, one possible plot could be that Sans somehow gets hit by a strange magic spell. As a girl, he/she might struggle to fit into the existing relationships in the Underground. For example, Papyrus might be really confused at first but then tries to be supportive. And Sans (as a girl) could also have different interactions with Frisk, like maybe a more motherly or sisterly bond.
It could be another monster who is jealous of Sans' new form. This monster might feel that Sans is getting special treatment or opportunities because he's now a human. So, this jealous monster tries to find a way to either turn Sans back into a skeleton against his will or make his human life miserable by causing trouble for him in the human world, like spreading false rumors about him.
Well, one possible plot could be that the other characters in the story are initially shocked. Papyrus might be super confused but also determined to take care of his 'baby' brother. They could go on little adventures where Papyrus has to protect Sans from various threats in the Underground while also trying to figure out how to turn him back to normal.
Well, the plot could start with a young mechanic who is always around cars. One day, while working on a very special vintage car, there's a strange energy surge. The next thing he knows, he is the car. He experiences life from a different perspective. He can't speak in the traditional sense but tries to convey messages through his headlights and horn. He discovers he has new senses, like being able to detect mechanical problems in other cars just by being near them. He goes on a journey to find a way to turn back into a human while having many encounters with different people and cars along the way.
One possible plot could be Sonic accidentally getting hit by a strange energy beam during a battle with Dr. Eggman. This beam could have some sort of mutagenic effect that slowly starts to transform him into a human. As he turns into a human, he has to deal with the loss of his super speed at first. He then has to adapt to living as a human while still trying to protect his friends and the world from Eggman's new threats.
In such a fanfiction, there could be a lot of conflict. Underfell Sans is from a more hostile AU than Undertale. He might initially be distrustful when meeting the characters from Undertale. For example, he could misunderstand their kindness as weakness or a trick.
Well, it could start with a new conflict arising between humans and monsters due to a misunderstanding. Maybe a human accidentally breaks a crucial monster relic, and the monsters think it was a deliberate act of aggression. Then, both sides start to gather their forces. The main character, perhaps a half - human half - monster, tries to bridge the gap and stop the war before it gets out of hand.
Well, one possibility could be that the six human souls somehow break free from their captivity within the game's lore. They might then team up with the monsters in the Underground to find a way to co - exist peacefully. Maybe they use their combined powers to change the rules of the Underground, allowing for a more harmonious relationship between the two races.