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.
When writing the fanfic, think about the reactions of the other characters. The dragons might not care much, but the human characters will. Some might be accepting, while others could be hostile. Also, consider how Hiccup's goals and dreams change. If she was focused on becoming a great Viking warrior before, as a girl, she might also want to fight for the rights of female Vikings in the village.
Don't forget to add some unique elements to the story. Maybe Hiccup discovers new talents or skills as a girl. She could be better at healing, for example, or have a special connection with a different type of dragon. And make sure to keep the essence of Hiccup's character intact. Even though she's now a girl, she should still be brave, intelligent, and kind - hearted.
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.
One possible fanfic could be that Hiccup's transformation into a Dragon Toothcup is the result of a magical experiment gone wrong. Maybe a wizard in Berk was trying to create a new form of dragon - human bond and accidentally merged Hiccup with a special dragon toothcup. As a Dragon Toothcup, Hiccup could have unique abilities like being able to sense the emotions of dragons more acutely and communicate with them on a deeper level. His friends would be both shocked and intrigued, and they would have to figure out how to reverse the transformation.
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.
With his human friends, it would be really tough. They might be scared of him at first. Astrid, for example, would be conflicted. She was attracted to Hiccup as a human, but now he's a dragon. She would have to overcome her fear and try to find a way to communicate with him. And Hiccup would feel isolated from his human friends, longing for the connection they once had.
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.
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.
You need to create a plot. Maybe start with an event that triggers the use of dragon magic. For instance, a new threat to Berk that only dragon magic can defeat. Then build the story around Hiccup's role in this. He could be learning to control the magic, or trying to find a way to amplify it. Also, describe the dragon's personality and how it uses its magic. Is it a powerful but clumsy dragon? Or a small but very precise one? All these details will make your 'dragon magic hiccup fanfic' interesting.
First, create interesting characters. Decide on the nature of the dragon, like its appearance and abilities. For the yaoi aspect, develop a relationship dynamic between the male characters. As for the 'hiccup' part, it could be used as a cute or comical element in their interactions. For example, every time one character gets nervous around the other, he hiccups.
Stoick would be quite surprised and a bit worried. As a father, he'd take on the responsibility of caring for his baby son. He might be a bit clumsy at first, not used to dealing with a baby again. The other Vikings would have mixed reactions. Some might find it funny, while others would be more concerned about how to reverse the situation and get their leader back to normal.
First, you need to come up with a unique plot for Hiccup's transformation into a weredragon. Maybe it's a magical curse or a hidden ability that gets triggered. Then, focus on how this change affects his relationships and adventures in the story.