Well, perhaps the story could be about Hiccup getting magically transformed into a baby Night Fury. Then he has to learn how to survive as a dragon in a world that is both familiar and strange. He might have to rely on Toothless, his friend, to protect him and teach him the ways of being a Night Fury.
The plot could also center around the mystery of how Hiccup turned into a baby Night Fury. There could be an ancient magic or a curse involved. Hiccup, in his new form, starts to have strange dreams or visions that give him clues about how to turn back. Along the way, he discovers new things about the Night Furies that he never knew before, like some hidden abilities or a secret place that only Night Furies know. And he has to use these new - found knowledge and abilities to break the spell and return to his human form.
Obviously, Hiccup and Toothless are the main ones. Astrid could also be in it, maybe she tries to figure out what happened to Hiccup and how to turn him back. There could also be other Vikings who are either scared or intrigued by the baby Night Fury that used to be Hiccup.
One possible plot could be that Hiccup, after some magical or scientific experiment gone wrong, transforms into a Night Fury. At first, he's terrified and confused. His dragon Toothless tries to help him understand his new form. Hiccup then discovers new abilities like enhanced flying speed and the power to shoot more powerful blasts. The other Vikings are initially scared of him, but he eventually shows them that he's still the same Hiccup inside, just in a different form. And he uses his new form to protect the village from a greater threat that normal dragons and Vikings couldn't handle alone.
Well, it could be that Hiccup is cursed by an evil sorcerer who wants to turn him into a dragon as a form of punishment. But as he turns into a dragon, he finds that he has a new connection with the dragon world. He starts to see things from a dragon's perspective and uses this to bring peace between humans and dragons in a whole new way.
His relationship with Toothless would change drastically. Instead of being the rider, he becomes like a little sibling or a young apprentice to Toothless. Toothless would have to be more protective and patient with Hiccup.
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.
A power - hungry Viking who sees Hiccup's transformation as a threat to his own authority could be the main antagonist. He might try to capture Hiccup in his Night Fury form and use him for his own evil purposes, like trying to create an army of transformed creatures.
Maybe it could be about Hiccup's journey to prove himself worthy of being the king. He might face challenges from other dragons or Vikings who don't believe in him at first. Then he uses his unique bond with Toothless and his understanding of dragons to overcome these difficulties and finally take the throne.
One possible plot could be that Hiccup's transformation into a baby dragon is the result of a magic spell gone wrong. He then has to figure out how to reverse it while experiencing the world from a dragon's perspective. He might make new dragon friends who help him understand his new form and search for a cure.
The fanfiction could focus on the baby having some special abilities. Since Hiccup and Astrid are both strong characters, their baby might inherit unique traits. It could be able to communicate with dragons in a special way. And the story could revolve around how they discover and nurture these abilities while also trying to keep their baby safe and normal within their community.