You could have a story where characters from How to Train Your Dragon find themselves in the world of Game of Thrones and have to navigate its politics and dangers.
You might find crossovers where characters from both universes interact in unexpected ways. Maybe a dragon rider shows up in Westeros or a Game of Thrones character discovers a dragon egg in the land of Berk.
To write such a fanfiction, you could begin by deciding on a central conflict that ties in with the themes of both. Maybe a powerful enemy threatens both Berk and Westeros, forcing them to work together. Also, pay attention to maintaining the tone and style of both franchises to make the crossover feel seamless.
You could start by taking the magical creatures and adventure from 'How to Train Your Dragon' and combine them with the complex politics and power struggles of 'Game of Thrones'.
In Game of Thrones fanfiction, you might lose your dragon by making a foolish decision in battle or underestimating an enemy's strategy.
One possibility is a tale where a Time Lord gets involved in the Game of Thrones' political intrigues and uses time travel to change the course of events. Or imagine if a White Walker was accidentally transported to the Doctor Who universe and caused chaos.
Berk could write a fanfiction about the adventures of the characters after the events of the movie. Maybe exploring new lands or facing new villains.
There are various fanfictions that imagine Vikings interacting with the world of How to Train Your Dragon. Some might focus on friendship, while others could explore adventure or romance.
You can find all sorts of storylines. Some might focus on the characters' adventures after the events of the movies, while others might explore alternate universes or prequels.
One popular theme is the further adventures of Hiccup and Toothless. Writers often explore new lands and challenges that they might face. For example, they might encounter a new type of dragon with unique abilities.
It might be a story where the characters have to deal with the dragon's frequent and smelly farts, leading to all sorts of adventures and challenges.