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.
It might be a gift from a powerful dragon in Naruto's world. In some fanfictions, there are hidden dragons that see potential in Naruto and bestow upon him the power of dragon scales, which could be a symbol of strength and protection.
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.
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'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.
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.
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.
The dragon scales might enhance Hiccup's physical abilities. For example, they could make him stronger, so he can better handle his weapons during battles. Also, they may improve his balance when he's riding Toothless, allowing for more daring maneuvers.
In some fanfiction, Hiccup might kill a dragon because it was corrupted by a dark force. He may have tried to save it first, but when all else failed, he had to end its life to protect others. This would show his growth as a character, from someone who once befriended a dragon to someone who has to make tough choices to safeguard his people and the world he lives in.
The plot could center around a competition. Hiccup is invited to Merida's land for a friendly contest. They both enter different events. Hiccup in a dragon - riding competition and Merida in an archery tournament. Along the way, they learn from each other's strengths and overcome their own weaknesses. They also have to deal with a cheating competitor who is trying to undermine the spirit of the competition.