Often, in such a fanfiction, the story develops around the chaos that his transformation brings. Since Inuyasha is now a child, his usual brash and confident self is replaced with a more innocent and sometimes naïve version. This can lead to some comical situations, like trying to fight with a smaller body and less strength. His friends have to keep him out of trouble more than before, and there could be a plot where they search for a way to turn him back to normal.
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.
Well, one possible plot could be that his friends have to take care of him. Naruto might be really confused at first but then becomes super protective. Sakura could use her medical skills to make sure baby Sasuke is healthy. And they all have to figure out how to turn him back to normal while also dealing with the chaos that a baby brings.
Well, perhaps an enemy uses a magic spell to turn Zoro into a baby as a way to weaken the Straw Hat Pirates. At first, the crew is in chaos trying to deal with a baby Zoro. But as time passes, they see that baby Zoro has a pure sense of determination. He still tries to train, albeit in a very clumsy and cute way. And in the end, they go on an adventure to find the antidote to turn him back, with lots of comical and touching moments along the way.