Another idea is a crossover with 'Avatar: The Last Airbender'. The Trollhunters could meet Aang and his friends. They could combine their powers to defeat a common enemy that has the ability to disrupt both the balance of the four elements and the Troll world's magic. For example, they could fight a dark spirit that is draining the magic from the Troll world and using it to spread chaos in the Avatar world.
Begin by creating an outline. Outline the main plot points, like how the characters will meet, what their initial reactions will be, and how they'll overcome their differences to fight the common foe. If you're doing a Trollhunters and 'Lord of the Rings' crossover, you might start with the Trollhunters being transported to Middle - earth. Outline how they'll interact with the hobbits, dwarves, and elves. Then, start writing your first draft, don't worry too much about making it perfect at first.
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.
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.
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.
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.
The Trollhunters series has a really engaging world and characters. Fans love to explore different scenarios and relationships that might not be fully explored in the original show, so they turn to fanfic.