Clary Fray would be a great character to include as she is one of the main characters in the Shadow Hunter world. Her skills and determination can add a lot to a crossover story.
If it's a crossover with the Avengers, Thor could be an interesting addition. His god - like powers and different view on the world compared to the Shadow Hunters would create some great interactions.
In a crossover with the Mortal Instruments series itself (more of an internal crossover), Jace Wayland. His complex personality and his relationship with other Shadow Hunters can be explored further when combined with elements from other stories or universes.
Alucard from "Hellsing" would be a great addition. His overpowered nature and complex personality would contrast well with D's stoicism. They could have an interesting dynamic, either as allies or enemies depending on the plot.
In a 'Shadow of War Crossover Fanfiction' with 'Star Wars', Luke Skywalker would be a good character to include. His journey from a farm boy to a Jedi Knight could be contrasted with Talion's path. And Han Solo's roguish charm could add some humor to the story. If it was a crossover with 'Harry Potter', Hermione would be great. Her intelligence and knowledge of magic could interact in interesting ways with Talion's skills. Dumbledore could also be included for his wisdom and power.
Sora from Kingdom Hearts could be a fun addition. His positive attitude and his ability to use keyblades and magic would be a different take in the dark Castlevania setting. He could bring a sense of hope and light to the story. Also, Cloud Strife from Final Fantasy VII. His Buster Sword skills and his experience in fighting against powerful foes like Sephiroth could translate well in the battles against the lords and demons in Castlevania Lords of Shadow. His stoic personality would also create an interesting dynamic with the more emotional characters in the game.
The Hunter themselves are of course very popular. Their mysterious nature and badass fighting skills make them a central figure in any crossover. They can bring their signature weapons like the Saw Cleaver or the Hunter Axe into new settings.
In a Dorohedoro crossover with 'One Punch Man', Saitama would be an interesting addition. His overpowered nature would contrast with the often - more - gritty and complex power levels in Dorohedoro. Another good character could be Guts from 'Berserk'. His dark and brooding personality would fit well in Dorohedoro's gloomy atmosphere. Also, Edward Elric from 'Fullmetal Alchemist' could bring his knowledge of alchemy and his quest for truth to the Dorohedoro world.
In vegito crossover fanfiction, characters like Jiren would be great. His strength and stoic nature would make for some intense interactions with Vegito. Also, Android 18 could be included. She has her own unique set of powers and a strong will. Her presence could add some interesting dynamics, especially if she has to team up with Vegito against a common enemy. And don't forget Frieza. His evil nature and thirst for power always make for an exciting addition to any Dragon Ball - related story.
Kenny McCormick (Mysterion) could cross paths with Batman. Batman's detective skills and high - tech gadgets would make for an interesting dynamic with Mysterion's more 'underground' superhero style. They could work together to solve a mystery that spans Gotham City and South Park.
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.