The plot might be that Pitch has a new plan to create chaos. He discovers that turning Jack into a baby pooka can disrupt the order of the magical world. Once Jack is transformed, he is found by a group of friendly forest spirits. These spirits don't know who Jack really is at first but take him in. As time passes, Jack starts to show some of his old abilities in small ways, like being able to create a tiny bit of frost. This catches the attention of the Guardians who then start to investigate. They eventually figure out it's Jack and have to fight Pitch to get the antidote to turn Jack back.
Jack's personality would likely become more innocent and naive. As a baby pooka, he might be more curious about the world around him without the usual wariness he has as his normal self. He could be more easily frightened by things that he wouldn't normally be afraid of as an older Jack. Also, he might be more impulsive in his actions, just going for things that catch his eye without thinking much about the consequences.
The plot could center around Jack Frost being cursed into his baby pooka form by a mischievous winter spirit. He then has to find a way to break the curse. Along the way, he meets other magical beings who either help or hinder him. For example, a snowman he created in his normal form might come to life and guide him to a magical crystal that has the power to reverse the curse.
Perhaps in this form, Jack Frost is befriended by a young human child. The child doesn't realize he's Jack Frost and just thinks he's a really cool and strange little animal. They go on adventures together in the winter wonderland, with Jack Frost using his powers in small, unnoticeable ways at first.
One interesting story could be about Jack Frost teaching Baby Pooka how to create unique ice sculptures. In this fanfiction, Jack Frost uses his powers over ice and snow to show Baby Pooka the beauty of his creations. Another could be a story where they team up to save their magical realm from an encroaching darkness. They use their individual abilities, Jack's cold powers and Baby Pooka's special skills, to fight off the evil forces.
Well, one possibility is that he could be taken in by a kind family. They might not know who he really is and just think he's an ordinary baby. They would care for him, feed him warm milk, and keep him cozy in a little crib. As he grows up, he might start to show signs of his magical powers, like making little ice crystals form on his crib railing when he's excited or sad.
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.