Ron and Uryu might be paired. Ron has his own insecurities but also his moments of great courage, much like Uryu. They could bond over their skills in combat, Ron with his magic and Uryu with his Quincy powers.
Harry Potter and Ichigo Kurosaki are very likely to be paired. They are both the main protagonists in their respective series. Harry has his strong sense of justice and magical powers, and Ichigo has his determination and Soul Reaper abilities. They could form a great partnership to fight against common enemies.
Harry Potter and Nate Ford could be a likely pair. Harry's experience with facing great evils and his leadership skills combined with Nate's strategic mind in planning heists would make for an interesting dynamic. They could learn from each other's methods of problem - solving.
Percy Jackson and Carter Kane would be a great pair. Percy is used to the Greek mythological world and its challenges, while Carter has a deep understanding of Egyptian magic and mythology. Their different perspectives could lead to some interesting teamwork, like when they face a monster that has aspects of both Greek and Egyptian mythology, such as a Sphinx with the ability to use water attacks like a Greek hydra.
Yoko from Gurren Lagann and Hermione Granger could be paired. Yoko is a strong and independent female character, and Hermione is brilliant and resourceful. They could team up to face challenges, with Yoko providing her combat skills and Hermione her vast knowledge of magic. Their combination could be a force to be reckoned with in the fanfic.
Luna and Neville would be another possible pair. They are both somewhat outcasts in their own ways in the Harry Potter world. In the fanfic, when the bottle lands on them, it could lead to a really sweet and heartwarming moment. They could bond over their shared experiences and unique perspectives on magic, and perhaps develop a deeper relationship.
Lovi (Lovino Vargas from Hetalia) and Levi (from Attack on Titan) could be a likely pair. They both have that tough - exterior and are often seen as no - nonsense characters. Their interactions could be full of bickering and grudging respect. Another possible pair could be America (Alfred F. Jones from Hetalia) and Eren (from Attack on Titan). They are both quite headstrong and have a strong sense of justice, so they might team up to fight the Titans and try to bring about change in the world.
Mabel and Marco could be an interesting pair. Mabel is all about fun, friendship, and having a good time. Marco is more of a practical and responsible character. Their interactions could be really comical as Mabel tries to get Marco to loosen up and have more fun, while Marco tries to keep Mabel in line when things get too wild.
Harry Potter and Captain America are likely to be paired. Harry is used to leading a group against dark forces, and Captain America is a natural leader too. Their leadership styles could complement each other. Also, they both have a strong sense of justice.
Harry Potter himself could be a likely focus. His status as the Boy Who Lived and his powerful magical abilities would make for an interesting reincarnation story. He could be reincarnated into the Bleach universe and become a powerful Soul Reaper or something unique within that world. Maybe he'd use his Gryffindor courage in new and different ways.
Voldemort would also be a key character. His obsession with immortality and his actions that led to many deaths in the series are fundamental to the theme of life and death. In a crossover, he could be pitted against new threats or forces that have different views on death, which would make for an interesting story.