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.
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.
Well, the theme of survival is quite common. In many fanfictions, the female Night Fury has to fight for her survival in a harsh environment. Also, the theme of self - discovery is there. She tries to understand her own powers and identity. Family can also be a theme, whether it's about finding her lost family or creating a new family of her own with friends she makes along the way.
One idea could be that a Night Fury somehow gets transported into the Harry Potter universe. Maybe it crashes into the Forbidden Forest and Harry and his friends discover it. They could try to figure out how to communicate with it and use its abilities to their advantage against the dark forces in Hogwarts.
Family is a theme that sometimes shows up. It could be about the idea of Toothless and the female Night Fury starting their own family or how they fit into the dragon family structure. Overcoming challenges is also common. Maybe they have to face a new threat together, or deal with misunderstandings between them. And of course, the theme of discovery, like discovering new places or abilities as a pair.
The plot could be about Naruto being transported to the world of How to Train Your Dragon and stumbling upon a Night Fury. Maybe he tries to befriend it using his ninja skills.
One possible plot could be that Harry discovers a hidden Night Fury in the Forbidden Forest. He realizes that the dragon has some sort of connection to an ancient magic in the wizarding world. So, he tries to protect it from those who would want to use it for dark purposes. Along the way, he learns new spells and techniques from the Night Fury, which he then uses to fight against Voldemort's forces.
One interesting 'little night fury fanfiction' could be about a little night fury's adventure in a magical forest. It discovers new friends and overcomes various challenges like finding its way through enchanted mazes.