Another theme is adventure. Fanfictions usually take Hiccup on various quests, whether it's to discover new lands, find a lost dragon species, or solve a mystery related to the dragons. These adventures test his skills as a dragon master. There are also stories that focus on the theme of self - discovery. Hiccup is constantly evolving in the official story, and fanfictions often continue this journey, exploring how he learns more about himself, his powers, and his place in the world of dragons.
You could explore their relationship in times of crisis. Say Hiccup gets injured and his dragon has to take care of him. It could bring food, keep him warm, and protect him from other threats. This would show the deep bond of loyalty and care between them. Also, showing how they train together for new skills can further the exploration. They might face difficulties at first, but through patience and trust, they master the new skills, strengthening their relationship.
A rogue dragon rider who has different ideas about how to treat dragons. He might see them as tools for power rather than friends like Hiccup does. This rider could be causing trouble for Hiccup and his tribe by stealing dragons or using them to attack other tribes.
Good characterizations. When the author gets Hiccup's character right, as well as the dragons. They should feel like the ones from the movies but also have some new aspects. For instance, if Toothless has a new power that he discovers in the fanfic. Also, the relationships between the characters should be well - developed.
I really liked 'A Dragon's Bond'. It delves deep into the relationship between Hiccup and Toothless. There are a lot of emotional moments, like when they face a really tough challenge together and their bond is put to the test. It also has some cool new dragon designs.
One popular hiccup the dragon master fanfiction is 'A New Beginning for Hiccup'. It shows Hiccup facing new challenges in a land far from Berk and forming an unexpected bond with a rare dragon species.
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.
One interesting story could be about Hiccup exploring a new, uncharted island with Toothless and encountering a rare, magical dragon species. They have to figure out how to communicate with it and earn its trust.
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.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of HTTYD fanfictions. Just search for 'Hiccup dragon master' and you'll get a bunch of results.