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.
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.
Well, start by really understanding Cloudjumper's personality and traits. Make the story exciting and full of adventure. Also, keep the tone in line with the original movie.
First, get a clear idea of Cloudjumper's character traits. Then, think of a compelling plot that fits within the How to Train Your Dragon universe. Maybe involve other characters from the franchise for added depth.
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.
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.
In my opinion, 'Flight of the Firewyrm' is among the best. It has a really exciting plot where a new type of dragon, the Firewyrm, is discovered and the riders have to figure out how to deal with it. 'Friendship Beyond Boundaries' is good too. This fanfic explores the friendships not just among the human characters but also between humans and dragons in a more profound way. And 'The Dragon Riders' Reunion' is a must - read. It reunites the old dragon riders in a situation where they have to face a new threat together.
Once upon a time in the land of Berk, Hiccup was flying on Toothless when a strange magic portal opened. Out of it came Merida from the land of DunBroch. Hiccup was startled but curious. Merida was amazed by the dragons. They decided to team up. Hiccup showed Merida how to ride Toothless and in return, Merida taught Hiccup some of her archery skills. Together they went on an adventure to save both their lands from a common enemy that was using dark magic.
Some characters might be amused. For example, Hiccup could find it interesting to see different interpretations of his adventures in HTTYD 2. He might laugh at the creative liberties taken in the fanfiction.