Well, one possible plot could be that a villain uses a new type of quirk -altering device on Deku. As a child, he might have to rely on his basic knowledge of heroics and his natural determination. His friends would be confused at first but then quickly realize it's still Deku inside. They'd have to protect him from other villains who might see this as an opportunity to get rid of the future symbol of peace while he's vulnerable.
In a 'Deku gets turns into a child' fanfiction, perhaps there's an accidental quirk malfunction during a training exercise. Deku, now a child, has to deal with the loss of his current physical strength and skills. However, he could show his friends and mentors a different side of his hero - making journey. For example, he might display a purer form of his original dream to be a hero, which could inspire them even more. Also, it could lead to some really cute and funny moments, like trying to fit into his old hero costume which is now way too big for him.
The plot could be that Deku is doing some sort of experiment in the school lab when something goes wrong. A chemical reaction or a malfunctioning device causes him to turn into a baby. All Might hears about it and comes to visit. He feels a sense of responsibility as Deku is his successor. The other students are both amused and worried. They start to learn about taking care of a baby, like feeding, changing diapers, and making sure he sleeps well. Meanwhile, the teachers are researching how to reverse the transformation, and they suspect that it might be related to some ancient quirk theory that they need to dig deeper into.
Well, one possible plot could be that Deku accidentally drinks a strange potion created by a villain. This potion has the side - effect of changing his gender to female. At first, he's shocked and confused. But then, he realizes that he can use this new form to his advantage in hero work. For example, he might be able to infiltrate a female - only villain group more easily. He also has to deal with his friends' reactions, some are surprised while others are supportive and help him adjust to his new body while still being a hero.
One possible plot for this fanfiction could be that Deku is facing a powerful and unknown enemy that has the ability to nullify his powers. He's in a desperate situation when Kurogiri shows up. Kurogiri, for reasons unknown to Deku at first, uses his portal - creating abilities to rescue Deku. It could turn out that Kurogiri has a connection to Deku's past that he's not aware of. Maybe Kurogiri was once influenced by someone who saw the same kind of potential in Deku that he himself had when he was younger, and that's why he saved him.
In the 'deku gets turned into a child fanfiction', Deku is the main focus, but his friends like Uraraka and Iida would be key characters. Uraraka might be especially kind and motherly towards him. Iida could be strict in making sure Deku follows the right path. And villains like Shigaraki could see this as an opportunity to target Deku in a new way, so they would also be significant characters in this scenario.
One possible plot could be that Zoro, turned into a girl, has to deal with the different reactions of his crewmates. Some might be shocked, some might tease him. And he has to learn to adapt to his new physical form while still maintaining his tough fighting spirit.
In the 'deku gets turned into a child fanfiction', it could lead to some really interesting storylines. Maybe he would see the world in a whole new way, with the innocence of a child. His interactions with his mentors could become more like a child - adult relationship where they have to guide him more. Also, his enemies might underestimate him at first, which could create some unique battle scenarios.
One possible plot could be that Percy, as a child, has to navigate his newfound smallness while still having the memories of his older self. He might struggle to convince his friends that he is actually Percy, and they initially think it's some sort of trick. Then, he could use his knowledge from his older self to help them in a new and unexpected way, like solving a mystery that the younger versions of them couldn't figure out on their own.
Perhaps in the fanfiction, when Touma turns into a child, he discovers a new side of the world he didn't notice before. As a child, he has a different perspective. Maybe he befriends other children who are in some sort of magical or dangerous situation, and his child form has unique powers to help them. The story could be about him growing and learning while in this child form.
One possible plot idea could be that they meet at school for the first time. Maybe Deku is being bullied and Uraraka stands up for him. This would start their friendship early on. They could then discover their shared dreams of becoming heroes together.
Well, he could be held by a villain group who wants to find out the secrets of his power. Maybe they'll try to study his One For All quirk.