In a rotg and httyd crossover fanfiction, perhaps the Guardians could come to Berk to protect it from a new threat. Pitch Black might team up with some dragon hunters, and the httyd characters along with the Guardians would have to work together. Hiccup could learn some new skills from the Guardians, like how to be more stealthy from Jack Frost, and the Guardians could be in awe of the dragons' intelligence and strength.
The characters can interact in many ways. For example, Jack Frost can use his powers to play with the httyd dragons. He could create ice sculptures for them to play with or use his ice slides to race the dragons in the sky. Hiccup could show Jack Frost how to ride a dragon, and in return, Jack could teach Hiccup some of his tricks for moving around quickly and quietly.
One idea could be to have Toothless meet Jack Frost. Maybe Jack could try to ride Toothless and they could have an adventure together exploring the skies and the wintery landscapes. It would be cool to see how their different powers interact, like Toothless' fire against Jack's ice.
With Jack Frost, his character can be developed by showing his reaction to the dragons. He might be fascinated by them at first but also a bit scared. His relationship with Toothless could be a key part. Maybe Toothless doesn't trust him immediately, and Jack has to earn that trust. This would show his determination and his ability to connect with different beings.
Well, some characters might be amused. They could find the creative interpretations in the fanfiction quite interesting and might even laugh at the funny or unexpected scenarios. For example, Toothless from HTTYD might be curious about how he is portrayed in the fanfictions.
In a really out - there view, if we consider the idea of the characters 'watching' as a metaphor for how the original creators might think the characters would react, then perhaps the creators could be influenced by that thought. But again, this is a very different way of looking at it and not really the characters themselves influencing the fanfiction.
One popular rotg crossover fanfiction might be the one where the characters from ROTG cross over into a modern-day city setting. It adds an interesting twist as they interact with the normal humans and face new challenges.
A common theme is the exploration of different types of magic. Since both worlds have their own magical elements, fanfictions often delve into how these magics interact. For example, how Jack Frost's power over ice compares to Elsa's.
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.
One great example could be a story where Jack Frost from Rotg teams up with the Avengers. Maybe he uses his ice powers to help them in a battle against a common enemy like Loki who has somehow gained control over a powerful ice - related artifact. Jack could freeze Loki's magic for a short time, giving the Avengers an advantage.