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.
In a fairy - tale world, a princess is cursed by an evil witch and turned into a baby. The whole kingdom is in chaos as they try to find a solution. The fairies in the forest come together to use their magic to protect the baby princess from further harm and work on breaking the curse. They face many challenges along the way, like dealing with the witch's minions who are trying to stop them.
Maybe there was a magical mishap. A powerful wizard was trying to cast a spell on another dimension but it backfired and hit Supergirl instead, turning her into a baby. In the fanfiction, Kara's friends like Lois Lane could try to take care of her in a more human way since she has lost all her powers as a baby. They would have to keep her identity a secret too.
One possibility could be that a powerful spell was cast on the superheroine by a villain. Maybe the villain wanted to weaken her and thought turning her into a baby would be the best way. In the fanfiction, the superheroine's friends would then have to figure out how to reverse the spell while also protecting the now - helpless baby version of her.
In a fanfic about Hermione turning into a baby, it could be a part of a time - travel plot. The time - travel magic somehow affects Hermione and reverts her to a baby. Her intelligence as an adult might still be somewhat present in her baby form, leading to some comical or interesting situations. For example, she might try to communicate complex ideas using baby sounds and gestures, and her friends would be left trying to decipher what she means.
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.
One popular 'baby deku fanfic' might be those that explore his early days with his unique quirk in a really cute way. For example, some fanfics could focus on how he discovers his power as a baby and how his family reacts to it.
It could be the classic villains from the original series like All For One. He might be interested in baby Deku's potential quirk. Another could be Shigaraki, who might see baby Deku as either a threat or an opportunity to disrupt the hero world in a new way.
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.
As far as I know, not really. Most traditional stories focus on more common transformations like a man turning into an animal or a different human form but not specifically into a baby girl. There could be some very new and emerging stories in the fan - fiction world perhaps, but these are not widely known or considered 'famous' in the general sense.