In a 'super heroine turns into a baby fanfiction', it could start with the heroine investigating a mysterious place. There, she accidentally activates a device that changes her into a baby. Her powers are still there but in a very unpredictable way as she doesn't have the control or understanding she had as an adult. Her friends in the hero community have to deal with her new form. Some might be good at taking care of her, while others struggle. And they all need to find a solution which could involve consulting with a very old and wise hero who has seen all kinds of strange things in his long career.
Wonder Woman could be a great choice. Imagine her with all her strength and abilities suddenly being turned into a baby. It would be quite a shock for her Amazonian sisters and the other heroes she works with. Her lasso of truth and her super strength would be very difficult to control in her baby form.
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.
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.
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 possible plot could be that a magic spell gone wrong in Berk turns Hiccup into a baby. Toothless, being his loyal friend, tries to protect the baby Hiccup from the chaos that ensues. The other Vikings are confused at first but then start to take care of the baby Hiccup in their own clumsy ways, leading to some comical situations.
Well, perhaps it could be about a magical event that causes Ash to transform into a baby. Then his friends have to take care of him while also trying to find a way to reverse the transformation. Maybe Pikachu has to protect baby Ash from other Pokémon who might mistake him for an easy target.
Maybe there's a new villain in the kingdom who has a device that can transform enemies into harmless forms, like turning Luigi into a baby. The villain wants to use this to stop Luigi and Mario from interfering with his evil plans. Mario has to protect baby Luigi from the villain's minions while looking for a way to break the transformation. This could involve exploring new areas of the kingdom and meeting new characters who might have the knowledge to reverse it.
Well, it could be that Edward is cursed by a powerful witch. The witch might be angry with him for some reason. And so, he turns into a baby. His friends then have to figure out how to break the curse while taking care of baby Edward. They might go on a journey to find a special herb or a magic item.
One possible plot could be that Ginny is hit by a magical spell that turns her into a baby. Her friends then have to figure out how to reverse the spell while taking care of baby Ginny. They might face challenges like finding the right ingredients for an antidote and dealing with Ginny's baby - like needs such as feeding and putting her to sleep.
One possible plot could be that a magical spell is cast on Simba by an envious rival. This rival, perhaps a forgotten outcast from the pride, sees Simba's position as king and decides to play a cruel trick. Simba suddenly finds himself in a baby's body, with all his adult knowledge but none of his physical strength. His friends and family are confused at first. Nala might be especially protective, like she was when Simba was a cub the first time. Simba has to figure out how to communicate his situation to Rafiki, the wise old mandrill, who then sets off on a journey to find a way to reverse the spell. All the while, Simba is learning new things about his kingdom and the animals in it from this new, baby - like perspective.