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.
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.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. Just type in 'HTTYD' in the search bar and then sort the results by popularity or rating. Another way is to join HTTYD - related fan groups on social media platforms like Facebook or Reddit. People in these groups often share and recommend their favorite fanfics.
The characters in 'httyd' would likely have complex reactions to the future in the fanfiction. Hiccup, for example, has always been a forward - thinking character. He would likely be intrigued by the possibilities the future holds, whether it's in terms of dragon - human relations or new inventions. However, he would also be concerned about protecting his tribe and the dragons. Astrid would be more practical. She would assess the potential threats and benefits. The dragons themselves, such as Toothless, would react based on their bond with their human counterparts. If they sense that Hiccup is positive about the future, they might be more open to it, but if there are signs of danger, they would be on high alert.
There is a 'brave httyd fanfic' called 'Alliance of Heroes'. In this fanfic, the main characters from 'Brave' find themselves in Berk and team up with Hiccup and his friends. They face a common enemy and it's really interesting to see how their different skills and personalities blend together in the story.
The plot could involve the Vikings of Berk discovering a magical link to the world of DunBroch. Hiccup, being the curious and adventurous type, decides to explore. He meets Merida and they quickly realize that there are threats in both their worlds that seem related. Maybe a dark force is trying to use the power of dragons and some ancient magic from DunBroch. Together, they must combine their skills, Hiccup's dragon - riding and knowledge of dragons, and Merida's archery and understanding of the magic in her land to defeat this common enemy.
A great 'HTTYD Star Wars' fanfiction might involve Hiccup and Toothless somehow getting transported to the Star Wars galaxy. They could end up helping the Rebel Alliance. For example, Toothless' speed and stealth could be used in battles against the Empire. Or perhaps there's a fanfiction where the dragon - riding culture of Berk in 'HTTYD' influences the way some of the Star Wars species view combat and cooperation. It could add a whole new dimension to the Star Wars lore.