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.
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.
Jim Lake Jr. from Trollhunters is an obvious choice. His journey as a Trollhunter and his growth throughout the series make him a very dynamic character. In a crossover, seeing him interact with the characters from 3Below, especially with his sense of justice, would be really interesting. Another interesting character would be Aja from 3Below. She is a strong - willed princess who has her own set of skills and knowledge. Her leadership skills and her ability to adapt to new situations could create some great storylines when paired with the Trollhunters.
Well, I think Aja is really popular. She has this great energy and her alien powers are really cool. Writers can focus on her relationships, like her friendship with the humans or her bond with Krel. Krel is also a top pick. His inventions and his role as the tech - savvy one in the group can be the center of many stories. Also, Varvatos Vex is a character that many like to write about. His loyalty and his warrior - like nature can add a lot of action and drama to a fanfiction.
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.
One idea could be that the characters from Trollhunters and 3Below have to team up to fight a common enemy that is threatening both their worlds. Maybe an ancient evil that has the power to destroy Arcadia and Akaridion - 5. Another could be a story where the technology of 3Below is combined with the magic of Trollhunters to solve a mystery that has been plaguing both species for centuries. For example, a mystery about a long - lost artifact that has the power to reshape their universes.
Maybe the technology from '3 Below' gets mixed up with the magic in the Trollhunter universe. For example, Aja and Krel's advanced tech could malfunction in the Troll world and create some unexpected chaos. The Trollhunters would then have to help them fix it while also dealing with their own troll - related problems.
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.
In addition to them, there are some other significant characters. For instance, Strickler, who has his own motives and secrets. He is a complex character who sometimes helps and sometimes causes trouble for the Trollhunters. And there are various troll characters with different personalities and abilities, each playing a role in the story's development, like Draal, a powerful troll who also has an important relationship with Jim.
One great theme is friendship. The characters in the show have strong bonds, and fanfictions often explore how these friendships develop further or face challenges. For example, a fanfic might focus on how the main characters support each other during a difficult mission.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. These platforms have a large collection of fanfictions related to various shows, including 3 Below Trollhunters of Arcadia. Just use the search bar and type in relevant keywords like '3 Below Trollhunters of Arcadia' to narrow down the results.