Well, perhaps in the fanfiction, Sirius is experimenting with some new magic and it goes wrong, resulting in Harry becoming a baby. Sirius could then embark on a journey to find ancient magic books or consult with old wizard friends to reverse the transformation. Along the way, he might have some heart - warming moments with baby Harry, like when Harry first smiles at him, which makes him even more determined to turn Harry back.
One possible plot could be that Sirius accidentally casts a spell that turns Harry into a baby. He then has to take care of Harry in this new form while trying to find a way to reverse the spell. He might face challenges like hiding Harry from the wrong people, and also dealing with Harry's baby - like needs such as feeding and changing diapers.
One possible plot could be that Sirius manages to escape with Harry undetected. He could then raise Harry in secret, perhaps in a wizarding community on the outskirts. Sirius would teach Harry all about his parents and the wizarding world, but also keep him hidden from Voldemort's remaining followers. Harry would grow up with a strong sense of family and justice, having Sirius as his sole parental figure.
In this fanfiction, it could be that a magical accident occurs during a potion - making class. Harry accidentally drinks a wrong potion which transforms him into a baby. Dumbledore then decides to keep Harry at Hogwarts, with the teachers taking turns to look after him. Meanwhile, Neville could be the one who discovers a clue about how to reverse the transformation by observing some strange behavior of magical plants in the school's greenhouse.
Well, one possible plot could be that the other characters in the story are initially shocked. Papyrus might be super confused but also determined to take care of his 'baby' brother. They could go on little adventures where Papyrus has to protect Sans from various threats in the Underground while also trying to figure out how to turn him back to normal.
One possible plot could be that the magic in the world goes haywire. Natsu is caught in a spell that turns him into a baby. His friends in Fairy Tail are shocked at first. Lucy might take on the role of a caretaker, trying to figure out how to reverse the spell while also dealing with a baby Natsu's unpredictable behavior like randomly using small bursts of fire magic that are more like cute little puffs now.
Well, the plot could involve his teammates having to take care of him. They might struggle at first because Kuroko as a baby can't communicate in the same way. Maybe they have to figure out his needs like when he's hungry or sleepy. And they could also try to keep it a secret from the other teams to avoid any distractions during games.
The story could start with some sort of strange experiment gone wrong that turns Junkrat into a baby. The rest of the crew is shocked at first. Mei could use her scientific knowledge to try and find a way to reverse the process. Meanwhile, Reinhardt might tell stories to baby Junkrat about their adventures, as if he can understand. And Lucio could play some soft music to keep the baby calm.
Well, at first, Sirius would be in a state of shock and panic. But then he'd have to step up. Their relationship would shift from one of equals (in a sense, as Harry was growing up) to a caretaker - cared - for dynamic. Sirius would have to learn about Harry's likes and dislikes as a baby. He'd sing lullabies to him, comfort him when he cries. This new experience would make their bond stronger in a very different way. Harry might not remember all of it when he's turned back, but there would be a subconscious change in how he feels towards Sirius, a deeper sense of security and love that stems from those baby - days when Sirius was his whole world.
Well, one possible plot could be that Jimmy's latest experiment goes wrong and he is turned into a baby. His friends, like Carl and Sheen, would be really confused at first. They might try to figure out how to reverse the process. Meanwhile, Jimmy as a baby would still have his super - intelligent mind, but be limited by his baby body. So he could be using baby - like actions to communicate his scientific ideas to his friends, like babbling and pointing at things he needs for a new experiment to reverse his condition.
The plot could center around the mystery of how Ciel turned into a baby demon. There could be a search for a way to reverse it. His allies, like the Phantomhive servants, would be in a tizzy trying to keep him safe and figure out what happened. Ciel himself, despite being a baby, might still have some of his old memories and try to communicate in his own baby - like way, which could be really cute yet frustrating for those around him.
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.