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.
In a crossover with "My Neighbor Totoro", Totoro would be an amazing character to include. Coraline could stumble upon Totoro's forest and be both awed and a bit confused by this huge, friendly creature. The Catbus could also be involved, transporting Coraline to different parts of this new world that combines elements of her own and Totoro's world.
For a crossover with "Peter Pan", Peter himself would be an obvious choice. He could take Coraline to Neverland, and she could have adventures with the Lost Boys. Tinker Bell might be a bit jealous of Coraline at first but then they could become friends. Captain Hook could also be a menacing presence in a new way in this crossover, especially if he sees Coraline as a threat to his plans in Neverland.
For a crossover with Doctor Who, the Doctor could be an interesting addition. He could bring his time - traveling abilities and vast knowledge to the Shadow World. And from the Shadowhunter side, Jace Wayland would be a good character to include. His charm and combat skills would make for some interesting dynamics with characters from other universes.
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.
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.
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.
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.