If it's a crossover with 'Final Fantasy VII', Cloud Strife would be a great character to include. His sword - fighting skills and his cool demeanor would be a great contrast to Joel. Tifa could also be there, and she could bond with Ellie. Tifa could teach Ellie some self - defense moves, and they could all work together to survive in the post - apocalyptic world of The Last of Us.
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.
For a crossover with a post - apocalyptic world, characters like Mad Max could be involved. He might see the homunculi as either allies or threats in the harsh new world. In a crossover with a cyberpunk world, characters like Neo from 'The Matrix' could be interesting. The homunculi could be part of the complex digital and physical world that Neo navigates, and he could help them understand the true nature of their existence in this new, high - tech world.
In a crossover with 'Fullmetal Alchemist', Edward Elric could be an interesting addition. His scientific knowledge and his search for the Philosopher's Stone could interact with the strange alchemy - like aspects of Rea's resurrection in Sankarea. He might be both fascinated and appalled by what has happened to Rea.
Definitely include Bill Cipher from Gravity Falls as he is the main villain in Weirdmageddon. Dipper and Mabel Pines are also essential as they are the main protagonists in the Gravity Falls story. If it's a crossover with the Over the Garden Wall universe, Wirt and Greg would be interesting additions. Wirt's cautious and somewhat gloomy nature could contrast with the more adventurous Gravity Falls characters, while Greg's innocence could add a different dynamic.
Crash Bandicoot would be a great addition. His goofy and fearless nature would contrast well with Spyro's more noble and magical persona. He could bring some comical moments to the story while also being a strong ally in battles.
If you're doing a crossover with 'Attack on Titan', Eren Yeager would be interesting. His determination to fight Titans could be combined with Negi's mission to protect his students. Mikasa, with her combat skills, would also be a great addition. And in a crossover with 'My Hero Academia', Deku's growth as a hero could be paralleled with Negi's growth as a teacher and mage.
The Powerpuff Girls would be great. Unikitty could join forces with them to fight crime in Townsville. Her magical powers combined with the girls' superpowers would make for an unbeatable team.
You could also include Ultraman. Ultraman's high - tech powers and Mothra's natural, almost mystical powers would create a great contrast. They could learn from each other and fight side by side against a horde of invading space monsters.
In a springtrap crossover fanfiction, characters like the Purple Guy could be included. Since he has a connection to Springtrap, it would make for an interesting story. You could also include some original characters, like a brave adventurer who tries to stop Springtrap. And perhaps a mad scientist who created or is trying to control Springtrap.
If the crossover is with a magical girl series, a character like Hanekawa Tsubasa could be interesting. Her intelligence and her own experiences with the supernatural in bakemonogatari could make her a bridge between the two different universes, perhaps learning new magical powers or sharing her own knowledge.