Goku would be an obvious choice. Having father - son adventures in a crossover setting, like in a different universe or a new realm, could be really exciting. They could face challenges that require both their power levels and unique abilities. It would also be a great opportunity to explore their relationship further in a new context.
Gohan would be a great character to pair with Future Trunks in a crossover fanfic. Their friendship and combined power could make for an interesting dynamic. In a crossover, they could support each other in a new and unfamiliar world. Another good option is Vegeta. Since Future Trunks is his son, their relationship could be explored further in a different setting. They could also team up to fight stronger enemies. Also, Bulma could be paired with Future Trunks. She could use her scientific knowledge to help them in a crossover situation, like building new gadgets or finding a way home.
In a crossover with the Avengers, Thor would be an interesting character to pair with Madara. Thor has his godly powers and Mjolnir, while Madara has his Sharingan and immense combat skills. The clash between Thor's lightning and Madara's fire - based jutsu would be epic. Another good pairing could be with Batman. Batman's strategic mind against Madara's cunning and power would make for a great dynamic. Batman would have to rely on his gadgets and intelligence to outwit Madara.
Arya Stark from 'Game of Thrones' could also work. It would be a very different kind of crossover. Arya's skills as an assassin could be compared to Galen Marek's combat abilities. They could find themselves in a situation where they need to rely on each other's skills in a strange new world that combines elements of both their universes.
Lucy Heartfilia could be paired with Zancrow. She has a different set of abilities and a more diplomatic personality. Her interactions with the hot - tempered Zancrow could lead to some interesting character development for both. She might try to reason with him or use her celestial magic in unexpected ways against his fire attacks.
Link from The Legend of Zelda could be paired with iskandar. Link is a skilled fighter, and iskandar could learn from Link's agility and stealth. In return, iskandar could teach Link about large - scale battle tactics. Also, Alucard from Hellsing would make an interesting pairing. Alucard's dark powers and iskandar's noble leadership would create a contrast, and they could either be enemies or allies depending on the story.
Ken Kaneki could be a great character to pair with Yamori. Their different outlooks on life and their shared experiences in a harsh world could create some intense and thought - provoking interactions.
Issei would be a great choice as he is from the same universe and has a strong connection with Rias already. Their relationship can be further explored in a crossover, especially if they are facing new and different challenges together.
Goku Black could be an interesting character to pair with Gogeta. Their power levels are both high and their personalities are quite different. Goku Black is more of a villainous character, so the dynamic between him and Gogeta, who is a hero, would be full of tension. Another interesting character could be Saitama from One Punch Man. The contrast between Gogeta's complex power - up system and Saitama's one - punch - kill ability would make for an exciting crossover. And then there's Naruto. His ninja skills and determination paired with Gogeta's power would create a unique combination.
In crossover fanfiction, pairing Josuke Higashikata with Light Yagami from Death Note would be an interesting choice. The contrast between Josuke's heroic nature and Light's more morally ambiguous actions would create a lot of tension. Another good pairing could be Eren Yeager from Attack on Titan. Eren's drive to protect his people and Josuke's ability to use Crazy Diamond in combat could result in a powerful alliance. Also, Saitama from One - Punch Man. Saitama's overpowering strength and Josuke's unique Stand abilities could lead to some comical and action - packed scenarios.
Captain America is an obvious choice. Their long - standing relationship in the original Marvel universe provides a rich backdrop for a crossover. You can explore their friendship or the times when they were on opposite sides. Black Widow is also great. She has a history with the Winter Soldier, and they can team up or have some sort of conflict in a crossover. Another good character to pair with him is Hawkeye. Hawkeye's skills as an archer and his different moral compass compared to the Winter Soldier can create interesting dynamics.