Light Yagami from Death Note. The mind games between Light, who has the Death Note, and Jotaro with Star Platinum would be intense. Light would try to find a way to outwit Star Platinum's power while Jotaro would be trying to stop Light's plans.
Definitely Darth Vader. His presence would create a lot of tension. He could be on a mission to hunt down the Rebels crew, and it would be a battle of wits and strength as they try to outmaneuver him.
Well, from the 'Star vs the Forces of Evil' universe, obviously Star and Marco are must - includes. They are the main characters and their dynamic is really interesting. In a crossover with 'Gravity Falls', Dipper and Mabel would be great additions. Dipper's curiosity and Mabel's wackiness could create some really fun interactions with Star and Marco. They could all investigate some strange phenomena that combines the mysteries of both Gravity Falls and Star's dimension.
One popular character to include is Giratina. Even though it's a Legendary Pokemon, it can be given a more complex personality in fanfiction. Maybe it has a reason for being so feared and banished to the Distortion World that goes beyond what is shown in the games. Also, Fantina, the Gym Leader of Hearthome City, with her unique style and love for Ghost - type Pokemon, can be a great addition to a fanfiction. And don't forget about Maylene, the Fighting - type Gym Leader. Her journey to become stronger and her relationship with her Pokemon can be explored in depth.
Seven of Nine would be an exciting character to include. Her unique backstory and abilities could add a lot to a crossover. In a crossover with the Doctor Who universe, for instance, her Borg knowledge might be useful in dealing with the Cybermen. Picard is also a classic character. His diplomatic skills could be put to the test in a crossover with the Star Wars universe, negotiating with the Galactic Republic or the Empire.
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.