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.
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 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.
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.
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.
A really interesting plot twist is when the villain in the fanfiction turns out to be a character who was initially thought to be an ally. For example, in a story where the Vikings and Arendelle are working together, a seemingly helpful Arendelle noble is actually in league with a dragon - hunting group from httyd. It throws everyone into chaos and makes for a great read.
One great piece of httyd and frozen fanfiction is 'A Dragon's Adventure in Arendelle'. It combines the magic of the dragons from httyd with the icy world of Frozen. The story has Toothless befriending Elsa and they go on a journey to save both Berk and Arendelle from a common threat.
One great 'frozen watches httyd fanfiction' could be the one where the characters from Frozen are transported to the How to Train Your Dragon world and they have to learn to ride dragons. It's really interesting to see how Elsa's powers interact with the dragons.