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.
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.
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.
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.
The plot could also center around the mystery of how Hiccup turned into a baby Night Fury. There could be an ancient magic or a curse involved. Hiccup, in his new form, starts to have strange dreams or visions that give him clues about how to turn back. Along the way, he discovers new things about the Night Furies that he never knew before, like some hidden abilities or a secret place that only Night Furies know. And he has to use these new - found knowledge and abilities to break the spell and return to his human form.
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.
The popularity of 'night fury hiccup x toothless fanfic' is mainly due to the strong bond between Hiccup and Toothless in the original franchise. Fans love to explore more of their relationship and adventures through fanfics.
Toothless' loyalty plays a big part. His loyalty to Hiccup makes him determined to save Hiccup no matter what. This loyalty has developed over time through their shared adventures and experiences.