One possible plot could be that Harry wakes up one day and discovers he has taken on Sans' form and abilities. He has to figure out what happened while also dealing with the confusion of his friends in the wizarding world. At the same time, he starts to understand Sans' world better and realizes there are similarities between the two. As he gets used to his new form and powers, he might use them to protect both the wizarding world and Sans' world from threats that are starting to emerge, all while trying to find a way to reverse the transformation and return to his normal self.
It could be about Harry being transported to Sans' world and having to adapt to its rules. Maybe he uses his magic in unique ways there.
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.
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, if Harry Potter had sans powers, he might be able to teleport around Hogwarts easily. This would make it much simpler for him to avoid danger or get to his classes on time. For example, when facing Voldemort, he could suddenly disappear and reappear behind him to gain an advantage in battle.
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.
Maybe the plot involves Sans and Frisk finding a strange magic that results in the baby. Then they have to figure out how to raise the baby in their unique world. Sans might be worried about protecting the baby from any threats, and Frisk would be more focused on the emotional growth of the baby. They could also face difficulties in teaching the baby the values of their world, such as the importance of determination.
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.
Well, it could be about Gaster suddenly reappearing in the lives of Sans and Papyrus. Maybe he has some important knowledge or a mission for them. For example, he might need their help to fix something in the Underground that's going wrong.
Harry might gain the ability to dodge attacks more easily. Sans has a certain 'instinct' for avoiding harm, and if Harry had this, it would be much harder for his enemies to land a hit on him. This would be really useful in duels, especially against powerful dark wizards. It could also make his Quidditch playing even more amazing as he could dodge bludgers effortlessly.
Maybe the drunk bunny is a lost soul in a strange dimension. Sans discovers the bunny and takes it under his wing. The fanfiction could then explore their relationship as the bunny sobers up gradually. For example, they could start to bond over shared experiences or interests. The bunny might also introduce Sans to some new things from its own world, creating an interesting cultural exchange within the story.
One possible plot could be Sans using his teleportation ability (from his original game) in the My Hero Academia world to outwit villains during a big battle. For example, he could suddenly appear behind a villain like Stain and disarm him before he even realizes what's happening.