A possible plot for this fanfic is that a dark force is spreading across the land, and it's affecting the dragons. Hiccup, in his quest to find a solution, comes across an old legend about a Dragon Toothcup. He decides to search for it, thinking it might be the key to saving the dragons. Once he turns into the Dragon Toothcup, he has to figure out how to use his new form to purify the dark force. Along the way, he meets other magical creatures who help him understand his new powers and the significance of his transformation.
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.
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.
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.
In the fanfic, Astrid could be part of a scientific experiment gone wrong. The experiment was meant to study dragon DNA, but something malfunctioned and she starts to transform. She then embarks on a journey to find a cure. Along the way, she discovers that being a dragon gives her a unique perspective on the world and she has to decide if she really wants to go back to being fully human.
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.
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.
It could be a dragon hunter who sees Hiccup in his dragon form and wants to capture him for study or to use his unique abilities for evil purposes.
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.
A plot twist could be that Hiccup's transformation into a dragon is part of a curse by an evil dragon sorcerer. The sorcerer's plan is to use Hiccup, now a dragon, to lead the dragons into a war against the humans. But Hiccup, with his strong will and knowledge of both humans and dragons, tries to find a way to break the curse and prevent the war.
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.
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.