One great 'dragon magic hiccup fanfic' could be about a dragon using its magic to cure Hiccup of a strange ailment. The dragon might have to go on a quest to find a special herb that is enhanced by its magic to make the cure.
You need to create a plot. Maybe start with an event that triggers the use of dragon magic. For instance, a new threat to Berk that only dragon magic can defeat. Then build the story around Hiccup's role in this. He could be learning to control the magic, or trying to find a way to amplify it. Also, describe the dragon's personality and how it uses its magic. Is it a powerful but clumsy dragon? Or a small but very precise one? All these details will make your 'dragon magic hiccup fanfic' interesting.
One possible fanfic could be that Hiccup's transformation into a Dragon Toothcup is the result of a magical experiment gone wrong. Maybe a wizard in Berk was trying to create a new form of dragon - human bond and accidentally merged Hiccup with a special dragon toothcup. As a Dragon Toothcup, Hiccup could have unique abilities like being able to sense the emotions of dragons more acutely and communicate with them on a deeper level. His friends would be both shocked and intrigued, and they would have to figure out how to reverse the transformation.
Sometimes Astrid can also be a main character in these fanfics. She's Hiccup's friend and fellow dragon rider. Her presence can add a different dynamic, like in a story where she and Hiccup have a disagreement about how to handle a dragon - related situation, but in the end they work together to solve the problem.
One interesting storyline could be Hiccup and his dragon exploring uncharted lands. They might encounter new dragon species and have to figure out how to communicate or coexist with them.
Well, in these fanfics, Hiccup being a ninja usually means he has to master the art of moving silently and quickly. He might have secret missions or have to fight against powerful foes using his newfound ninja abilities. And it could add a whole new level of excitement to the How to Train Your Dragon universe.
Hiccup is not useless. The fanfic might present him with difficult situations, but he always finds a way to contribute and make a difference. His unique skills and determination shine through.
There are many ways to approach such a fanfic. You could start by imagining what adventures Hiccup would have after his return and build the story around that.