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.
In a fanfiction scenario, Viggo might kidnap Hiccup out of jealousy. Hiccup has a special bond with Toothless, a very rare and powerful dragon. Viggo, who has always been obsessed with dragons, may be envious of Hiccup's connection. He kidnaps Hiccup to try and take Toothless for himself. He might think that by having Hiccup in his control, he can make Toothless obey him. He could also use Hiccup as a bargaining chip to get more dragons from Berk.
Well, Hiccup is very resourceful. He could use the environment around him to his advantage. If he is locked in a place, he might find a weak spot in the structure, like a loose board or a hole in the wall. And he could call for Toothless secretly. Toothless, being highly intelligent, could find Hiccup and rescue him. Also, Hiccup could befriend some of Viggo's minions and convince them to help him escape.
Some characters might be amused. For example, Hiccup, being the open - minded and curious one, could find it interesting to see how fans imagine different scenarios. He might even get some new ideas for his own adventures.
Fishlegs would likely have a very animated reaction. If he comes across some fanfiction that has really strange dragon facts, he'd be so shocked. He might start rambling on about how wrong it is according to his extensive dragon knowledge. His eyes would be wide and he'd be waving his hands around.
First, you need to have a clear idea of their characters and the setting. Then, think of an interesting plot that could involve both Hiccup and Viggo. Maybe start with a conflict or a mystery for them to solve together.
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.