Adventure is also a very common theme. Writers like to send the characters from both universes on quests or to face new and dangerous enemies. For example, they might have to battle a powerful dark force that is a threat to both the troll world and the world of dragons and Vikings. This allows for a lot of action - packed scenes and exploration of new terrains in the fanfiction.
There's a fanfiction called 'Bridging the Worlds' that I really like. In this story, the main characters from Trollhunters and How to Train Your Dragon find themselves in a strange in - between world. It's a detailed and well - written piece that shows how the different cultures and powers interact in a unique situation. The character development is also quite good as they learn from each other's ways.
One idea could be that the characters from 3 Below end up in Arcadia Oaks where the Trollhunters are. They could team up to fight a common enemy that threatens both their worlds. Maybe the Gumm - Gumm trolls somehow form an alliance with the bad guys from 3 Below's planet, and the combined forces of the heroes have to stop them.
One common theme is adventure. The reader often gets involved in exciting quests and battles in the troll world. Another is friendship. They form strong bonds with the trollhunter characters. And self - discovery is also there. The reader discovers new things about themselves while in this strange troll - filled world.
One common theme is the struggle for acceptance. The vampire characters often have to deal with being different in the HTTYD world, which is already filled with unique dragons and riders. Another theme is power. Vampire abilities can bring great power, and the fanfictions usually explore how the characters handle this power, whether it's for good or bad.
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.
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.
One common theme is the exploration of power. Since Jim is the heir of Merlin, there's often a focus on the new powers he gains and how he learns to control them. Another theme is identity. Jim has to reconcile his identity as the Trollhunter with this new identity as Merlin's heir. Also, friendship is a big theme. His friends' reactions and how they support or sometimes question him in this new role.
One common theme is the crossover adventure. Combining the magical elements of Miraculous Ladybug with the dragon - riding world of HTTYD creates exciting new storylines for characters to explore different lands and face new challenges.
Another theme is friendship. For example, when a character like Astrid is hurt, her friends rally around her. They not only help with physical wounds but also emotional ones. It shows how important friendship is in difficult times.
Another theme is adventure. Both Danny Phantom and HTTYD have elements of adventure in their original stories. In fanfiction, this can be amplified as the characters from both series go on new quests together. They might explore new lands in the HTTYD world or deal with new ghostly threats in Danny's world. Also, the theme of friendship is quite common. The characters from different universes form strong bonds and help each other out. For example, Toothless and Danny could form an unexpected friendship where Toothless helps Danny understand the concept of loyalty in a different way.