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.
Maybe there could be a new character introduced, like a wise old man or woman who has knowledge about such strange transformations. This character could guide Touma through his experiences as a child and help him understand the meaning behind this change. Additionally, any potential villains in the story would also be important characters as they could pose a threat to child Touma.
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.
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.
Well, one possible plot could be that Sakura is hit by a magic spell that turns her into a child. Her friends then have to figure out how to reverse the spell while also protecting her in her vulnerable state. They might take her to different magical places or consult with wise old sages to find a cure.
The plot could revolve around Corrin trying to find a way to reverse the transformation. Along the way, they could encounter various challenges. For instance, they might have to go on a quest to find a rare ingredient for a potion that could potentially turn them back. Their child form could make it difficult to traverse dangerous terrains or face powerful enemies. However, they could also make new friends among the younger characters in the Fire Emblem world, who could help them in their journey.
The antagonist could be a group of bullies in the story. They pick on Percy because he is now a child and seems weaker. They don't know he is actually Percy with all his heroic qualities. They constantly harass him, steal his things, and try to make his life miserable. This forces Percy and his friends to come up with strategies to deal with these bullies, which could include using Percy's unique skills even in his child form, like his ability to talk to sea creatures to get help when the bullies try to corner them near the water.
Another idea is that Misty is so in love with Vaporeon that she wishes she could be one. Her strong desire somehow triggers a strange phenomenon. She begins to change physically and mentally. Her hair might start to look like Vaporeon's fins, and she becomes more attuned to water energy. Along the way, she has to hide her transformation from her friends until she fully understands what's happening and how to control it.
One possible plot could be that the magic backfires on Voldemort during a powerful dark spell. As a baby, he has no memory of his previous self. The wizarding world is in chaos as they don't know what to do with this baby - voldemort. Some might want to kill him immediately, while others think he could be redeemed. Harry Potter, having defeated him once, feels a strange sense of responsibility and wants to make sure he doesn't grow up to be evil again.
A possible plot could focus on how Tails' daily life changes. As a girl, she might have to deal with different social expectations. Her hobbies and interests could shift slightly. For example, she might be more interested in certain types of fashion or more nurturing activities. Her relationship with Amy could also change, perhaps they become closer friends as they share more 'girly' things. And Tails' mechanical skills might be seen in a new light, as she uses them in a different way to solve problems in her new form.
Well, one possible plot could be that his friends have to take care of him. They might struggle at first, not knowing how to handle a baby Izuku. For example, Bakugo could be really frustrated but gradually learn to be gentle. Ochaco might be really good at soothing baby Izuku when he cries.