Ichigo and Zangetsu could be an interesting pair in a master - slave fanfic. Zangetsu, as the inner spirit of Ichigo's zanpakuto, has a sort of master - like relationship over Ichigo in terms of teaching him about his power. They could be in a situation where Zangetsu is more dominant in guiding Ichigo's growth, like a strict master with a somewhat rebellious slave.
Rukia and Renji could also fit. Renji has always respected Rukia and could be seen as her subordinate in a way. In a fanfic, this relationship could be explored further in a master - slave dynamic. Maybe Rukia has to train Renji in a more intense way due to some special mission requirements, and Renji has to follow her every command, leading to some interesting character development for both of them.
Aizen and Gin could be characters in such a fanfic. Aizen, being the powerful and manipulative character he is, could take Gin as his slave in a sense. Gin, who has his own motives and plans, has to act as the obedient slave while secretly working against Aizen. This would create a lot of tension and mystery in the fanfic.
One idea could be that a powerful Soul Reaper takes on a young and inexperienced one as a sort of apprentice in a master - slave relationship. The master could teach the slave about different zanpakuto techniques and the ways of the Soul Society. For example, they could go on missions together where the master protects the slave at first but gradually lets the slave take on more challenges.
Obviously, Ranma and Akane are prime candidates. Their relationship is central to the series, so a pregnancy between them would create a lot of drama and interesting storylines. Shampoo could also be involved in some way. Maybe she's jealous and tries to interfere, or she could be surprisingly helpful. Genma, Ranma's father, would also play a role. He might have some old - fashioned or comical ideas about the situation.
Obviously Skye herself is the main character. There could be a love interest who is the father of the baby. Her best friend could also be an important character, providing emotional support and practical help like going to doctor's appointments with her.
Ichigo would be a great main character. He's the protagonist in 'Bleach' and his strong sense of justice and powerful combat abilities would make him a key figure in the Hogwarts setting. He could interact with Harry Potter, and they could learn from each other's fighting styles.
A new human character could be introduced, like a military leader who is leading the invasion. He could have his own motives for wanting to take over the Soul Society, such as a thirst for power or a belief that humans should control all realms.
One potential character could be Sasuke. He has always been highly skilled with weapons and his Sharingan would give him an edge in mastering knife techniques. He has the determination and the combat skills to be a great knife master in a fanfiction setting.
Pinkie Pie might also be a good choice. Since Pinkie is already a bit of a wild card herself, her interaction with the Bride of Discord could either result in double the chaos or Pinkie Pie somehow taming the Bride's chaos in her own unique, zany way.
Well, obviously Naruto and Hinata as Shinachiku's parents. His friends could be the children of other Naruto characters, like Boruto's friends. Maybe the son or daughter of Shikamaru could be his strategic partner in some adventures. Also, the new generation of the Akatsuki could be potential villains in the fanfic.
Some potential characters could include Fleur Delacour. She's beautiful and has her own brand of magic. Cho Chang could also be part of it. And what about a new character, a mysterious witch from a hidden wizarding village? She could bring in new elements to the harem, like a different type of magic or a unique personality trait.
Obviously, Calvin himself as the main character. His parents could also play important roles, showing how their relationship with Calvin has evolved as he grew up.