Yes, there are likely some popular ones. For instance, some fanfictions that explore the relationship between Hiccup and Astrid during the time leading up to HTTYD 2 are quite popular. They might show how their relationship evolves and how they support each other in anticipation of the new threats in the second movie.
I don't have a definite answer. It might be a story of friendship, a quest for success, or a story filled with unexpected twists and turns. Without more context, it's difficult to pin down exactly what it is.
One interesting fanfiction could be about Toothless having a unique adventure during the time between the first and second movies that the other characters are unaware of until later in HTTYD 2. Another might focus on Hiccup's internal struggles as he anticipates the new challenges in HTTYD 2, all while still dealing with the aftermath of the first movie's events.
The other characters like Fishlegs would probably be really into it. He loves stories and knowledge, so seeing different takes on the HTTYD 2 story in fanfiction would be like a goldmine for him. He might start analyzing the details and comparing them to what he knows from the real events.
A 'httyd visual novel' is probably an interactive storytelling experience set in the HTTYD world. It might follow the general plot of the HTTYD movies or deviate to create unique stories. For example, it could show what happened before Hiccup met Toothless in more detail. The visual aspect would enhance the experience, with detailed drawings of Berk, the dragons, and the characters. It may also have different endings depending on the choices made by the player.
One idea could be to focus on how Astrid and Hiccup prepare for the baby. They might start building a new addition to their home on Berk. Hiccup could be super protective, while Astrid is determined to still be involved in dragon - related activities as much as she can until she can't anymore.
Yes, he might. Hiccup is a character from 'How to Train Your Dragon' series. If there is an interesting HTTYD 2 fanfiction out there, it's possible that someone could imagine Hiccup reading or being involved in it in some way in the context of fan - created stories.
It's hard to say if it's common. Since Valka is a fictional character, we don't really have data on her fictional reading habits. But in the context of the fandom, it could be seen as something that would be cool if she did, as it would show her interest in the fan - created content.