One possible plot is that Hiccup is on a quest to save a dragon in danger. During this quest, he accidentally stumbles upon an ancient dragon artifact. When he touches it, it starts a transformation process. As he becomes a dragon, his friends initially don't recognize him. He then has to find a way to show them who he is and together they face a new threat that only a half - human - half - dragon Hiccup can deal with.
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.
The plot might start with Hiccup having a strange encounter, like stumbling upon a hidden cave with a magical artifact. This artifact could be the key to his transformation into a dragon god. As he becomes more dragon - like in his powers, he could gain the ability to communicate with dragons on a whole new level, understanding their deepest thoughts and feelings. And he would surely have to use these powers to protect Berk from threats that normal human - dragon alliances couldn't handle.
There could be an old dragon that has been in exile for a long time. This dragon might see Hiccup as a threat to its plan of taking over the dragon kingdom. So it tries to stop Hiccup from fully becoming a dragon or tries to use his new form for its own evil purposes.
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.
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.
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.
Well, perhaps in the fanfiction, Hiccup is on a quest and stumbles upon a female dragon that is injured. He decides to take care of her. As she heals, she starts to show loyalty to Hiccup. They start communicating in their own special way, and she becomes his secret weapon in battles against other threats to Berk. She might also introduce Hiccup to a whole new world of dragon behaviors and habitats that he wasn't aware of before.
Maybe Natsu discovers a new magic spell that accidentally turns him into a dragon. He then has to figure out how to reverse it while dealing with the new powers and instincts.
Well, perhaps Hedwig could be magically transformed into a dragon. Then she might go on adventures, using her new dragon abilities like flying faster and breathing fire to protect her wizard friend from dark forces.
In this fanfiction, perhaps Hiccup turns into a female dragon because of an experiment gone wrong. His transformation would be a shock to Toothless especially. As a female dragon, Hiccup might have to adapt to a more nurturing role that is often associated with female dragons in the How to Train Your Dragon universe. He could start to understand the female dragon's perspective on protecting the young and the territory. And during his time as a female dragon, he might meet other dragons who become his allies in his quest to turn back into a human. There could be a lot of self - discovery and adventure along the way.
One possible plot could be that Hiccup discovers a special set of dragon scales that have unique powers. He decides to wear them for protection during his adventures. As he wears them, he starts to feel a deeper connection with the dragons, almost as if he can understand their thoughts better. This new connection helps him in battles and in finding new dragon habitats.