Well, one possible plot could be that Hiccup accidentally drinks a magic potion made by a mischievous wizard. As a girl, he (now she) has to deal with the different reactions of the people in Berk. Some are shocked, some are curious. And she has to learn how to adapt to the new body while still maintaining her heroic spirit, facing the same dragons but in a different physical form.
In a 'hiccup turns into a girl' fanfiction, the plot might start with a strange curse. Hiccup wakes up one day as a girl. At first, he's really confused. Toothless might not recognize him right away. Hiccup then tries to find a way to reverse it. Along the way, he discovers new things about himself. He might find that as a girl, he has different strengths and weaknesses. He could also have some interesting interactions with Astrid, who is both confused and intrigued by this new version of 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.
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.
A fanfic could explore how Hiccup's transformation into a girl affects his friendships. Astrid might react in a complex way. At first, she could be jealous or confused, but then they could form a stronger bond. Also, Hiccup's father, Stoick, might have a hard time adjusting to his son becoming a daughter, which could add a lot of emotional depth to the story.
First, you need to establish the reason for the transformation. It could be a magical spell gone wrong, or some kind of strange dragon - related phenomenon. Then, focus on how Hiccup adjusts to her new body. She'll have to deal with different physical abilities and limitations. For example, she might be physically not as strong as before, but more agile in some ways.
One idea could be that Hiccup turns into a dragon during a battle and uses his new form to surprise the enemy. He could fly faster and breathe a more powerful fire than any other dragon, leading his team to victory.
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.
One idea could be that Hiccup, after turning into a dragon, has to learn to communicate with his old dragon friends in a new way. Since he now experiences the world as a dragon, he might discover new dragon abilities and secrets that were hidden from him before. Maybe he has to find a way to turn back, but in the process, he helps the dragon world solve a big problem, like dealing with a new and dangerous enemy.
The plot could be that Hiccup finds a cursed dragon toothcup during one of his adventures. When he picks it up, he is immediately transformed. His friends think he's lost forever, but Hiccup in his new form tries to communicate with them. He discovers that he can hold dragon powers within the toothcup form. For instance, he can store and release dragon fire. With these new abilities, he helps Berk defend against a powerful enemy that has been threatening the village.
Astrid would probably be very surprised at first. She might be a bit jealous of Hiccup's new appearance as a girl, but also curious about how this change has affected Hiccup's personality. Stoick would be shocked and might be a bit overprotective. He'd have to adjust to the idea of his son being in a different form. Snotlout would likely make some inappropriate jokes at first, but then gradually come to accept Hiccup as she is. And Toothless? He might be a bit confused at first, but his bond with Hiccup would likely remain strong regardless of the physical change.
One possible plot could be that Cloud, after turning into a girl, has to adapt to a world that now views her differently. Maybe she struggles with newfound emotions and social expectations. For example, she might have been a tough warrior as a male, but as a girl, people underestimate her. She then has to prove herself all over again in battles or quests, while also dealing with her own internal confusion about her new identity.