His relationship with Toothless would become more complex. They would now be of the same species, which could lead to either competition or a deeper bond. Maybe Toothless would initially be confused about Hiccup's transformation but then accept him as an equal. They might communicate in new ways, without the need for Hiccup's human - made gadgets.
One idea could be that Hiccup wakes up one day to find himself in the body of a female Night Fury. At first, he's completely confused and tries to figure out how to control this new form. He might struggle with flying at first, but as he gets used to it, he discovers new abilities that come with being a female Night Fury, like a different type of sonic blast. The other dragons in the village are initially startled but then start to accept him in this new form.
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.
A power - hungry Viking who sees Hiccup's transformation as a threat to his own authority could be the main antagonist. He might try to capture Hiccup in his Night Fury form and use him for his own evil purposes, like trying to create an army of transformed creatures.
His relationship with Toothless would change drastically. Instead of being the rider, he becomes like a little sibling or a young apprentice to Toothless. Toothless would have to be more protective and patient with Hiccup.
His relationship with Toothless would be very different. Toothless might be more protective of him in his new form, as if sensing that Hiccup is more vulnerable as a female Night Fury. And Hiccup would have to communicate with Toothless in new ways, perhaps using more body language that is typical of female Night Furies.
Well, perhaps in the fanfiction, Hiccup turns into a Night Fury and loses his memory. He wanders off and gets caught up in a dragon - only territory. There, he has to learn the ways of the dragons all over again, but his human instincts still peek through sometimes. His friends from Berk set out to find him, and when they do, they have to figure out how to turn him back. Along the way, they learn more about the deep connection between Hiccup and dragons, and how his transformation might be related to an old prophecy.
You might create a story where Hiccup and Night Fury have to save their village from a mysterious threat. Or, explore a time when they get lost in a far-off land and have to find their way back home.
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.
Obviously, Hiccup and Toothless are the main ones. Astrid could also be in it, maybe she tries to figure out what happened to Hiccup and how to turn him back. There could also be other Vikings who are either scared or intrigued by the baby Night Fury that used to be Hiccup.