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.
Perhaps an evil mage who actually intended for the transformation to go wrong on purpose. They might want to use Sans' new female form to cause chaos in the Underground. They could be trying to manipulate Sans into doing bad things or using her to get to other important characters.
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.
For Frisk, they might see Sans in a new light. Instead of the cool and mysterious Sans they know, he's now a helpless baby. Frisk could become very protective, using their determination to keep baby Sans safe. And the monsters in the Underground would likely be more gentle and patient around baby Sans, changing the overall dynamic of the community.
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, 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' 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.
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.
One possible scenario could be that in this fanfiction, the change forces Kratos to view the world from a different perspective. As a girl, he might face different challenges and reactions from the other characters. Maybe the gods who once feared him now underestimate him, leading to some interesting power dynamics.
Perhaps in a parallel universe, Superman is born as a girl. This version of her grows up with different experiences and challenges. She could be more in tune with female issues in society and use her powers to fight for gender equality as well as the usual superhero stuff like saving the world from villains and disasters.