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.
One idea could be that Hiccup, after turning into a dragon, has to learn to communicate with his old dragon friends in a new way. Since he now experiences the world as a dragon, he might discover new dragon abilities and secrets that were hidden from him before. Maybe he has to find a way to turn back, but in the process, he helps the dragon world solve a big problem, like dealing with a new and dangerous enemy.
One idea could be that Hiccup discovers a new type of half - dragon that has unique abilities, like the power to control time briefly. This could lead to adventures where he has to use this new discovery to save Berk from an ancient threat that is about to be unleashed. Another plot could involve the half - dragon being a hybrid of a very rare and powerful dragon species, and Hiccup has to protect it from dragon hunters who want to use its power for evil.
One idea could be that Hiccup somehow gets transported to the world of The Dragon Prince. He could use his knowledge of dragons to help the characters there. For example, he might teach them new ways to bond with dragons or deal with dragon - related problems.
A more complex plot could be that a dark force is corrupting the dragons, and Hiccup, as the Dragon Master, has to find the source of this corruption. He has to use his skills and the bond he has with his dragons to not only stop the corruption but also save the dragons that have already been affected.
A possible plot could be that the Dragon King is being hunted by a new and powerful enemy. Hiccup, with his unique bond with dragons, sets out on a journey to find the Dragon King and offer his help. Along the way, he meets different types of dragons that have stories about the Dragon King. When he finally finds the Dragon King, they must work together to defeat this common enemy. Hiccup uses his intelligence and skills learned from his experiences on Berk, while the Dragon King uses its raw power and ancient knowledge.
Perhaps in the fanfiction, when Natsu turns into a dragon, he can communicate with other dragons in a way he couldn't before. This could lead to him uncovering secrets about his past and the origin of dragons in that fictional world. He might find out he has a special destiny related to the dragons.
One plot idea could be Hiccup's struggle to fully accept his half - dragon nature. He might face discrimination from both humans and dragons who don't understand his unique existence. Another could be his journey to discover his dragon heritage and the powers that come with it. Maybe he has to learn to control a special dragon - related ability that is a key part of his half - dragon identity.
One idea could be a story where Hiccup discovers a new type of dragon that has unique abilities. He then has to learn how to befriend and ride this dragon while also facing challenges from other dragon riders who are jealous of his new discovery.
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.
One plot idea could be Hiccup's struggle to fit in both the human and dragon worlds. He might be shunned by humans for his dragon - like features and not fully accepted by dragons either because of his human side. Another idea is that he discovers a unique power that only he, as a half - dragon half - human, has. This power could be the key to saving both his human village and the dragon population from a common threat.