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.
Obviously, Deku himself would be a main character even though he's turned into a baby. His friends like Uraraka, Iida, and Bakugo would also be main characters as they would be involved in taking care of him and trying to solve the problem.
One idea could be that in this fanfiction, Wolverine turns into a baby due to a powerful mutant's accidental spell. The X - Men then have to take care of him while also trying to find a way to reverse the transformation. They might discover new aspects of Wolverine's personality as a baby, like his unwavering determination is now shown through his attempts to crawl towards danger.
Well, one possible plot could be that a villain uses a new type of quirk -altering device on Deku. As a child, he might have to rely on his basic knowledge of heroics and his natural determination. His friends would be confused at first but then quickly realize it's still Deku inside. They'd have to protect him from other villains who might see this as an opportunity to get rid of the future symbol of peace while he's vulnerable.
Perhaps in the fanfiction, Deku turns into an adult due to a quirk malfunction. As an adult, he could struggle with his changed appearance and how his friends and family react. He might also find that his relationships with other characters, like Ochaco, change in unexpected ways as he is now an adult in a different stage of life.
A new villain could emerge who is specifically targeting adult heroes. Since Deku has now become an adult, he becomes a prime target. This villain might have a grudge against the hero society's adult leadership and see Deku as a symbol of that.
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.
One idea could be that a villain's new invention accidentally zaps Kim and turns her into a baby. Her sidekick Ron has to take care of her while also trying to find a way to reverse the effect. He might struggle with basic baby - care tasks like changing diapers and feeding her, all while keeping her safe from the villain who might want to take advantage of her weakened state.
One possibility could be that an akuma's power goes wrong and instead of attacking Chat Noir in a normal way, it turns him into a baby. Ladybug is then left to protect this baby Chat Noir while also trying to find a way to reverse the transformation. She might have to take him to Master Fu for help, all while hiding him from Hawk Moth who could potentially use this situation to his advantage.
One great fanfiction with this concept is 'Baby Dean's Adventures'. It has a really cute take on how the other characters deal with a baby Dean. They struggle at first but then it becomes a heartwarming story of protection and family.