North, also known as Santa Claus in Rise of the Guardians, could be an interesting character to pair with Hiccup. North has his workshop full of magical toys and elves. Hiccup could learn a lot about different kinds of magic from North. And North could be impressed by Hiccup's ability to tame dragons and might even want to have a dragon - powered sleigh for Christmas. Their interactions could be really fun to read about.
His character could also develop in terms of his understanding of different kinds of magic. He starts off knowing only about the magic related to dragons. But in the fanfiction, as he watches and interacts with the Guardians, he begins to understand their magic. This could lead to him becoming more open - minded and adaptable. He might start to think about how the different magics could work together to protect both his world and the world of the Guardians. And this new knowledge could also change how he approaches problems back in Berk. For instance, he could try to incorporate some of the Guardians' ideas or magic - like things into his dragon - training methods.
Some fanfictions might have a plot where Hiccup is mistaken for a new Guardian. He has to prove himself worthy while also trying to figure out how he can get back to Berk. In this process, he could form strong bonds with the other Guardians. He might teach them about the importance of friendship with different species, like how he is friends with Toothless, and they in turn could teach him about their powers and the magic that keeps the world in balance.
One interesting plot idea could be Hiccup befriending Jack Frost from Rise of the Guardians. Maybe they exchange their skills, like Hiccup teaching Jack about dragons and Jack showing Hiccup some cool ice tricks. Another could be that they team up to fight a common enemy that threatens both their worlds, like a dark force that can control ice and also enslave dragons.
The idea of good vs. evil is present in both. In Rise of the Guardians, it's the Guardians against Pitch Black. In Frozen, it's Elsa against her own fears which can be seen as an internal evil. In fanfiction, the Guardians could help Elsa face her fears. Jack Frost could use his carefree nature to show Elsa that she doesn't need to be so afraid. And the Frozen characters could help the Guardians in their fight against Pitch by bringing their own unique skills and determination.
You can start by looking on popular fanfiction websites like Archive of Our Own or FanFiction.net. Use the search function and type in 'Guardians Watches Rise of the Guardians'. Then, you can sort the results by popularity or rating to find the ones that are highly regarded by other readers.
Some Burgess characters could be a bit skeptical of the Guardians at first. They might think the Guardians are just stories. But when they actually meet them, they start to have a more hands - on interaction. Like a girl from Burgess who challenges Toothiana to prove she's real by showing her where all the teeth are hidden in Burgess. This leads to a fun and adventurous interaction between them.
Some fanfictions might take a different turn and introduce new threats to the guardians. Picture a story where a powerful dark force from another dimension comes to their world. The guardians then have to find new powers within themselves and work together in ways they haven't before to defeat this new enemy.
One interesting crossover could be the characters from Frozen interacting with the Guardians in a winter wonderland setting. Maybe Elsa's ice powers could be combined with Jack Frost's in some epic battles against a common enemy. For example, they could fight against Pitch Black together, with Elsa creating ice fortresses and Jack using his frost to make the ground slippery for the enemy.
One interesting plot twist could be when a character from Burgess who was thought to be just an ordinary person turns out to have a hidden power that is crucial in helping the Guardians. For example, a young girl might discover she can control ice like Jack Frost, which was unexpected and changes the whole dynamic of their fight against the bad guys.