As a girl, Harry's Hogwarts life would be full of changes. Firstly, his appearance would lead to different reactions from his friends. Some might be more cautious in their interactions, while others might be overly friendly in a different way. In the corridors, he might notice different looks from students. In the classroom, he may be grouped with other girls more often, which could affect his learning experience. When it comes to magical creatures, they might also react differently to him as a girl. His identity as a girl could also open up new opportunities for him to explore different aspects of magic that are more associated with female wizards in the fanfic world.
One challenge could be dealing with the change in how others perceive him. People who knew him as a boy might have a hard time adjusting, and he could face prejudice or misunderstandings. Another challenge might be adapting to the physical differences. For example, if there are differences in strength or body shape, it could affect his ability to do certain things like play Quidditch as he used to.
Harry might initially be shocked and confused. He's used to Hogwarts being his safe haven, so when it turns against him, he'll be at a loss. He may try to prove his innocence by finding evidence or confronting those who are spreading lies.
One possible reason could be misinformation spread by the Ministry of Magic. They might have spread false rumors about Harry being a threat, which influenced the Hogwarts community. For example, if the Daily Prophet printed false stories about Harry's supposed dark magic activities, it could turn some students and staff against him.
It could have a major impact. Harry's relationship with his teachers might also shift. Snape, who caused the transformation, might have some unexpected reactions to seeing Harry as a girl. Other teachers might treat 'her' differently too. And as for romantic interests, well, Harry might attract different people now. Some boys who never considered him before might start to notice 'her', which would create a whole new dynamic in his social life at Hogwarts.
Maybe it's a group of purist wizards who form a secret society. They are determined to 'purify' the wizarding world by getting rid of anyone or anything that is not a normal human wizard. Harry, as a monster girl, becomes their prime target. They use dark magic and underhanded tactics to try and capture or kill Harry.
One interesting plot could be how Harry, now as a girl, has to deal with the different social dynamics in Hogwarts. For example, she might face different expectations from teachers and peers. Maybe she forms new friendships with female characters in a way that Harry as a boy never did before.
One idea could be that Harry's transformation into a monster girl makes him/her (depending on how the transformation affects gender identity) discover a whole new world of magical creatures that were previously hidden from normal wizards. He/She could form unique bonds with these creatures and use their newfound abilities to solve mysteries in the wizarding world.
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.
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.