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.
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.
How about a crossover with the Percy Jackson series? The Trollhunter could meet Percy and his friends at Camp Half - Blood. Since both deal with mythical creatures, they could exchange knowledge about different mythologies and fight side by side against monsters that are drawn to the power of trolls and the demigods.
Next, write your story. Make sure to follow any guidelines the archive might have regarding grammar, formatting, and content. For example, if they don't allow excessive violence or inappropriate language, adhere to those rules. After writing, proofread your work carefully.
A cool plot could be that an interdimensional rift opens up and sucks in some of the characters from both shows. They find themselves in a strange in - between world where the rules of both their universes are mixed. The Trollhunters' magic doesn't work quite the same, and the technology from 3 Below is malfunctioning. They have to figure out how to get back to their respective worlds while also fending off strange creatures that are native to this new world.
Jim Lake Jr. from Trollhunters would be an obvious choice as he is the main Trollhunter. He could bring his knowledge of the troll world and his fighting skills to the crossover. Also, Aja from '3 Below' would be great. Her spunky personality and her alien powers would add a lot of fun and action to the story.
If it's a crossover with the Legend of Korra, Korra herself would be an excellent addition. She is a powerful bender and has a lot of experience in fighting evil. She and the Trollhunter could learn from each other's fighting styles. Asami could also be included, providing her engineering skills to help the Trollhunter with any technological needs in their battles against the common foes.
One interesting storyline could be Merlin using his magic to help the Trollhunters in a battle against an ancient and powerful troll enemy. Maybe he could use his knowledge of spells to create a magical barrier that protects the trolls and humans working together. Another could be a time - travel plot where a Trollhunter is sent back in time and meets Merlin, and they have to work together to change something in the past to save the future of both the troll and human worlds.