Batman would bring a different aspect to the fanfiction. His detective skills and strategic mind could complement Savitar's powers. He could be the one to figure out a complex plan to stop a villain that Savitar's speed alone can't handle. Maybe the villain has a hidden lair or a master plan that Batman has to decipher while Savitar provides the muscle and speed to execute the plan.
Superman could also be a great addition. His super strength and invulnerability combined with Savitar's speed would make for an interesting dynamic. They could face a powerful enemy that requires both of their unique abilities to defeat, like a being that can absorb energy and use it against them.
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 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.
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.
Well, if you're doing a crossover with "The Wizard of Oz", Dorothy would be a great addition. She and Coraline could compare their experiences of strange new worlds. The Scarecrow, Tin Man, and Cowardly Lion could also add some interesting dynamics, like the Scarecrow's new perspective on Coraline's world and how different it is from Oz.
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.
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.
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 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.
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.