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.
The characters could interact through a portal that links their two worlds. For example, Master Chief could end up in Berk and be initially seen as a strange and powerful warrior by the Vikings. Hiccup, being the curious and open - minded character he is, would be the first to approach the Chief and try to communicate. They could then start to exchange ideas about their respective worlds and threats they face.
Interaction could start with a misunderstanding. Maybe the Vikings see Rapunzel's long hair and think she's some kind of magical being they need to be cautious of. Meanwhile, Rapunzel might be scared of the dragons at first. But as they get to know each other, they realize they have a lot in common. Hiccup could help Rapunzel overcome her fear of dragons, and Rapunzel could show Hiccup how to be more in touch with his creative side, which he might not have explored much in his life as a Viking dragon trainer.
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.
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.
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.
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.
The characters can be integrated in various ways. Danny could befriend Hiccup as they both have a sense of being different from their peers. Danny is a half - ghost and Hiccup was initially seen as an outcast among the Vikings. They could share their experiences of not fitting in. Astrid could be curious about Danny's powers and train with him, learning new combat techniques that combine Viking fighting skills with Danny's ghostly moves.
The characters could interact through a portal that connects their two worlds. For instance, Applejack could enter the Undertale world and meet Toriel. Applejack's honesty and hard - working nature might impress Toriel, and they could exchange stories about taking care of others (Applejack on the farm and Toriel with the lost children).
They could interact through combat. For example, the Spartans might fight alongside the SG teams against a common foe like the Covenant or a powerful Goa'uld army. The Spartans' combat skills and advanced armor would be a great asset in the Stargate universe battles.
The characters could start off as enemies. The Horsemen might be seen as threats by the RWBY characters because of their intimidating appearance and their connection to destruction in the case of War. But then they could discover a common goal, such as stopping a powerful entity that is causing chaos in both worlds. They would then have to learn to work together, with the RWBY characters teaching the Horsemen about their world and vice versa.