First, you need to understand the characters and worlds of both HTTYD and Frozen. Then, think of an interesting plot that can bring the two together, like maybe a dragon from HTTYD somehow ends up in the Frozen world. Next, start writing the story, focusing on how the characters from both universes interact with each other.
One idea could be that the dragons in HTTYD are attracted to the cold of Frozen's world and they fly there. The people of Arendelle are initially scared but then they learn about the dragons from the dragon riders. Another idea is that Elsa's ice powers somehow have an effect on the dragons, like making their fire breath turn into ice breath for a short time.
First, you need to study the characters and settings of both 'How to Train Your Dragon' (HTTYD) and 'Frozen' well. For example, understand Toothless's character traits in HTTYD and Elsa's in Frozen. Then, think of an interesting plot that can combine the two worlds. It could be about a quest where the characters from both universes have to team up to face a common threat. Also, pay attention to the writing style, keep it engaging and easy to follow.
One popular theme is the combination of different kinds of magic. The ice magic from Frozen and the dragon - related magic in HTTYD. Another theme could be friendship across different worlds. Characters from HTTYD like Hiccup and those from Frozen like Anna forming an unlikely alliance. Also, the theme of discovery, as the characters from one world discover the wonders of the other world.
The idea of being different and having to find one's place in the world is also present in both. A fanfiction could explore how Elsa, with her ice powers, and Hiccup, with his connection to dragons, both feel like outsiders in their own communities at first. They could come together and help each other realize that their differences are actually their strengths. This could involve them teaching their respective communities about acceptance and the value of being unique.
In some fanfictions, Hiccup and Anna might team up. Hiccup's knowledge of dragons and Anna's adventurous spirit could lead them on a quest. They might encounter a new type of dragon that has been affected by Elsa's magic, and they have to figure out how to deal with it while also trying to get Elsa and Toothless to work together.
There are often crossovers with other fantasy universes. For example, in some 'httyd crossover fanfic', the Vikings from httyd meet the characters from 'Narnia'. The contrast between the two worlds, one with dragons and the other with magical creatures like centaurs and fauns, makes for a really engaging story. It can bring new adventures and challenges for the httyd characters.
Well, for a httyd transformers crossover fanfic, start with a strong concept. Maybe something like the Autobots land on Berk and have to figure out how to deal with the dragons. Focus on the interactions between the characters. The vikings' reactions to the giant robots and the Transformers' views on the dragons would be really interesting to explore. And make sure your writing style is engaging, using vivid descriptions to bring the crossover world to life.
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.
A 'frozen crossover fanfic' is a type of fan - created work. It typically combines the world or characters of 'Frozen' with those from another franchise. For example, it might cross 'Frozen' with 'Harry Potter'. So you could have Elsa using magic in Hogwarts or Anna making friends with Hermione. It's a creative way for fans to explore new scenarios and interactions that wouldn't happen in the original 'Frozen' story.