It could be Sam Winchester. He's known for his intelligence and his ability to research. If there's a time hiccup, he would be the one trying to figure out the cause and how to fix it using his knowledge of the occult and various historical events.
Dean Winchester is also a likely candidate. He's the more action - oriented of the two brothers. In a time - related fanfiction, he could be the one who has to physically fight off the threats that come as a result of the time hiccup. He might have to face old enemies in new situations or protect his loved ones from the chaos that time disruption brings.
Another possibility is Castiel. As an angel, he has a different perspective on time compared to humans. He could be the key to understanding the time hiccup. Maybe he has some knowledge or power that can be used to correct the time anomaly, but he also has to deal with his own angel - related issues and his loyalty to the Winchesters while trying to fix the problem.
The plot might revolve around a magical or supernatural event that causes a hiccup in time. This could disrupt the normal order of things in the Supernatural world. For example, it could make the usual rules of hunting demons or ghosts change, and the characters have to adapt quickly. They might have to team up with unexpected allies or face new and more powerful enemies that are a result of this time hiccup.
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 an old dragon that has been in exile for a long time. This dragon might see Hiccup as a threat to its plan of taking over the dragon kingdom. So it tries to stop Hiccup from fully becoming a dragon or tries to use his new form for its own evil purposes.
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 group of dragon hunters who don't believe in Hiccup's divinity could be antagonists. They think that dragons should be under their control, not Hiccup's. So they set out to capture all the dragons and overthrow Hiccup. They might use new and dangerous weapons to achieve their goals.
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.
Astrid could be one of the main characters. She might follow Hiccup to the marines to keep an eye on him or to have her own new adventures. Another character could be a tough marine sergeant who initially doubts Hiccup but later comes to respect him. There could also be a marine scientist who is interested in Hiccup's knowledge of dragons.
There could be a young girl from a far - off land who has a special bond with a rare type of dragon. She and Hiccup could join forces. She might have unique skills like being able to communicate with dragons without words. And also, an old hermit who has been living alone with his dragon for years. He has a lot of wisdom about the ancient ways of dragon riding and could help Hiccup on his travels.
It could be the thief of time themselves. We could get to know their motives, their daily life of stealing time, and their inner thoughts. It would be a very unique perspective.
Astrid would definitely be a main character. She could be the one who helps Hiccup readjust to life in Berk after rehab. She might be a bit hesitant at first, but then realizes that Hiccup has truly changed for the better.