There may be some extremely creative and boundary - pushing fanfiction out there with this premise, but it's not something that is typically found in the more well - known or widely - accepted "How to Train Your Dragon" fanfiction. Usually, fanfictions in this fandom focus on things like their adventures, friendship, and the development of their bond in a more traditional sense. So, while it's possible, it's not a common plot line.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
His friends could come to the rescue. Bakugo, for instance, might use his explosive power to break into Toga's hideout. He's always been competitive with Deku, but he also cares about him deeply.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
Toothless' origin story is quite interesting. He hails from a world filled with different dragon species. As a Night Fury, he was unique and had abilities like no other. His origin is marked by his wild existence before encountering Hiccup. Before that, he was just another dragon in a world where dragons were either hunted or used as weapons by humans. But when Hiccup entered his life, things changed. Hiccup saw something different in Toothless. This led to a relationship that defied the norms of their respective worlds. Toothless, who was once a feared and solitary creature, became a beloved friend and an important part of Hiccup's life and the entire Berk community.
Hiccup could be a main character. Since he is Toothless's close friend, he could help Toothless take care of the baby. He might use his human knowledge to build a safe place for the baby to live.
As of now, Deku is very much alive in the manga. The plot hasn't reached a point where he meets such a fate. Usually, main characters like Deku have significant story arcs and aren't easily killed off.