One idea could be that a powerful magic spell backfires on Ash. His friends then have to take care of baby Ash while trying to find a way to reverse the spell. They might face challenges like getting baby Ash to eat Pokémon food or stop him from toddling towards dangerous Pokémon.
In addition to the usual suspects, there could be some new characters introduced. For example, an old Pokémon trainer who has seen similar things before and can offer some advice on how to deal with baby Ash. Or a mysterious Pokémon that has the power to either help or hinder the process of getting Ash back to normal.
One possible plot could be that Ash wakes up one day as a girl and has to deal with all the changes in his daily Pokémon adventures. For example, he might have to adapt to new clothing styles and different social interactions. His Pokémon might also react in various ways to his new form.
It could be an old rival of the Malfoy family. Maybe a wizard whose family was wronged by the Malfoys in the past. This wizard has been biding his time, waiting for the perfect opportunity to strike. Turning Draco into a baby is his way of getting revenge on the family, making them look weak and powerless.
Perhaps it's a prank gone wrong. A group of Gryffindors, thinking they could teach Draco a lesson, cast a spell on him that they thought would just embarrass him. But instead, it turns him into a baby. Now, Hermione, being the brilliant witch she is, feels responsible for finding a cure. She starts researching in the library, while Ron is left to keep an eye on baby Draco. Ron has no idea how to take care of a baby, and there are lots of comical moments as he tries to feed Draco and keep him from crying.
Well, in such a fanfiction, probably Batman's sidekicks would have to take care of him. They'd be in a really strange situation, having to protect a baby Batman from the villains who might see this as a great opportunity to get rid of him once and for all.
Well, in such a fanfiction, Naruto would probably be really cute as a baby. His friends might be super confused at first. Sasuke might be a bit less brooding towards baby Naruto. And Iruka-sensei would probably take on a more parental role, making sure baby Naruto is safe and cared for.
Maybe it starts with a strange magical experiment gone wrong in Hogwarts. Harry is curious and gets too close, and suddenly he's a baby. The story could then focus on how the teachers, especially Dumbledore, deal with this situation. They might try to figure out if this is a new form of dark magic they haven't seen before. And Harry, as a baby, would have no memory of who he is, so it could also be about his friends trying to make him remember bits of his past life.
Perhaps an old enemy of Harry's family resurfaces. This villain might have a long - standing grudge against the Potters and sees turning Harry into a baby as a form of ultimate revenge. They could be using a spell that has been passed down through generations in their family, a spell that was created specifically to harm the Potters.
One possible plotline could be that his friends or allies have to take care of him in his baby form. They might struggle with things like feeding him, changing his diapers, and keeping him out of trouble.