Toothless is another great candidate. As Hiccup's dragon, he has a unique bond with his rider. In a past - meets - future story, he could be the one to sense the disruptions in time. He might also meet his descendants or ancestors in the future or past dragons, which could lead to some really interesting interactions. For example, an ancient dragon from the past might teach Toothless a long - lost skill that helps them in a future battle.
Astrid would also be a likely main character. In both movies, she plays an important role in Hiccup's life and the dragon - human relationships. She could bring her own skills and perspectives when the two movie storylines meet. And of course, Stoick might be there too, as his presence and leadership were important in the first movie and his legacy could play a part in the fanfiction that combines the two.
It could be a new villain from the future that somehow follows future Gohan back in time. This villain might be an evolved form of an existing enemy, stronger and more cunning. They could be after the secrets that future Gohan holds about the future technology or the power levels of the Z - fighters in the future.
One idea could be that an old enemy from the past resurfaces in the future, but with new and more powerful abilities. The dragons and riders have to combine their old - school knowledge with the new technology they've developed over time to defeat it.
Obviously, the main characters from 'the order' could be the main characters in this fanfiction. They are the ones who would be traveling to the future or interacting with future elements.
It could be a corrupted Gem from the past that was somehow locked away and now resurfaces in the future, angry at the changes that have occurred. This Gem might not understand the new world order and wants to return things to how they were.
The fanfiction might focus on how the relationships between the characters change when the two worlds of HTTYD and HTTYD2 collide. For example, Astrid might have to adapt to new ways of dragon riding she sees in HTTYD2, and this could cause some friction or growth in her relationship with Hiccup. It could also involve the dragons from the first movie learning new skills from the HTTYD2 dragons.
It could be Harry Potter's descendants. They might inherit his courage and magical abilities and be the main characters who navigate the encounter between Hogwarts and the future.
The main characters in the 'httyd meet their future selves fanfic' would be those who are crucial to the story of Berk. Hiccup, as the protagonist, is a given. Toothless, his dragon companion, is also essential. Astrid, being Hiccup's love interest and a strong warrior in her own right, would be there. And then there are the other members of the Dragon Riders like Fishlegs, who is known for his knowledge, Snotlout, who is full of bravado, and the Twins, who bring their unique brand of humor and chaos. All of them together form the core of the How to Train Your Dragon universe and would be the main characters in this fanfic.
One possible plot could be that future Gohan travels back in time to warn the past about an upcoming threat that was not properly dealt with in his own timeline. He might meet the past version of himself and try to train him to be stronger, sharing his knowledge of the future battles. Maybe he also meets the past versions of his friends and family, and has to convince them of the seriousness of the situation without revealing too much about the dark future he comes from.
The main characters are clearly Stoick and Hiccup. Stoick is Hiccup's father, and in this fanfiction, their encounter forms the central plot. Hiccup, as the character from the future, has likely developed and changed a great deal since Stoick last saw him.