It could be a dragon hunter who sees Hiccup in his dragon form and wants to capture him for study or to use his unique abilities for evil purposes.
A powerful and ancient dragon that sees Hiccup as an abomination for being a half - dragon, half - human. This dragon could try to destroy Hiccup to preserve the 'purity' of the dragon species.
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.
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.
Perhaps an ancient dragon that has been sleeping for centuries and wakes up to find Astrid turning into a dragon. This dragon sees Astrid as an abomination because she was not born a dragon but is becoming one through some unnatural means. So, it decides to hunt her down and destroy her.
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.
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.
Perhaps a jealous wizard. He was once friends with Astrid but when he sees her gain the power and beauty of a dragon, he becomes envious. He starts to cast spells to make Astrid's life as a dragon difficult and tries to steal her new - found powers for himself.
Toothless would be extremely confused yet instinctively protective. He would curl around Hiccup to keep him safe from any potential threats. Astrid would be a mix of emotions. She would be worried about Hiccup's well - being and also frustrated that she can't communicate with him in the normal way. The other Vikings would have diverse reactions. The ones who were skeptical of Hiccup's dragon - friendly ways before might see this as a sign that his ideas were wrong all along. However, those who trusted Hiccup would try to support him in this strange new situation. They might start looking for any magical items or wise people in the village who could help reverse the transformation.
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.
A powerful dragon rider from a far - off land who has a grudge against the Berk tribe (Hiccup's tribe) could be the main antagonist. He or she sees Hiccup and Heather's marriage as a symbol of the growing strength of Berk and decides to target them specifically to weaken the tribe. They might try to steal their dragons or spread false rumors about their marriage to create unrest among the people.
There could be Toothless. Even though Hiccup is now a dragon, his connection with Toothless from his previous human life might still be there. Toothless could play a crucial role in helping Hiccup adapt to his new form.