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.
Hermione Granger is also a strong candidate. Her intelligence and thirst for knowledge would translate well in the Bleach universe. She might be reincarnated as a researcher or a healer in the Soul Society, using her vast knowledge of magic (translated into some form of spiritual knowledge) to make a difference. She could also be a force for change within the traditional Bleach society.
Harry Potter himself is a likely candidate. He has endured so much throughout his life, from the death of his parents to the constant battles against Voldemort. Fanfiction could explore his post - war trauma and how he tries to lead a normal life.
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.
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.
Hermione is a very likely candidate. With her intelligence, she could easily start a business. For example, she might open a library - like establishment but with rare and restricted magical books that she has managed to collect through her research. She could charge a membership fee for witches and wizards to access these valuable resources.
Harry Potter himself is very likely to be the focus as he has green eyes. Fanfictions might explore different aspects of his character that were not fully developed in the original books. Another character could be Tom Riddle. Although his eyes are described as red in some instances, a fanfiction could play with the idea of what if he had green eyes and how that would change his story. Also, Ginny Weasley has green eyes and could be the focus, perhaps in a story about her life outside of her relationship with Harry.
Hermione and Betty would be likely to interact. Hermione's intelligence and thirst for knowledge could pair well with Betty's investigative nature. They could work together to solve a mystery that spans both the wizarding world and Riverdale.
The Weasley twins, Fred and George, are the most likely characters to pull pranks. They have a natural inclination towards mischief and are always coming up with new and creative ways to play jokes on their classmates and teachers. Their inventions, like the Skiving Snackboxes, are often used in pranks.
Definitely Snape. His complex character with his love for Lily and his role as a double agent has so much potential for exploration through memories. Harry himself is also interesting as his memories can uncover more about his family and his own growth. And Dumbledore, his past is full of mystery and his memories could reveal many secrets about his actions and motives.
Ichigo is often a popular crossover character. His strong and determined personality, along with his powerful spiritual abilities, make him stand out. He can easily interact with the more action - oriented parts of the 'Harry Potter' world, like dueling dark wizards or exploring dangerous magical places.
Lily and James Potter. Their resurrection would completely change Harry's story. He could finally have a proper family life and also gain more knowledge about his parents' past and their magical abilities. It would also add a new emotional depth to the story as they reunite with their son.