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.
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.