Well, a major theme would be oppression. Bellatrix is a cruel character, and Hermione being her slave would mean facing a great deal of oppression. There could also be a theme of hope. Hermione would probably hold onto the hope of escape or liberation. Additionally, the theme of the corrupting influence of power might be present. Bellatrix's power over Hermione could lead to her becoming even more sadistic and Hermione having to find ways to not be broken by it.
One common theme is the struggle for freedom. Harry, being in a'slave' role, would naturally strive for his freedom. Also, there could be a theme of unexpected alliances. Since it's a fanfiction, the author might create a situation where Harry and Bellatrix form an unlikely alliance against a common enemy, despite their initial relationship of master - slave.
Survival is a prevalent theme. Given Bellatrix's murderous and unpredictable nature, Hermione often has to fight for her survival in these fanfictions. It could be a physical fight, or a mental battle as she tries to outmaneuver Bellatrix in a dangerous situation.
A common theme is power struggle. Bellatrix is a powerful and dark witch, and Hermione is also very powerful in her own way with her intelligence and magic skills. So, there's often a back - and - forth as they try to assert their influence over situations or each other.
One common plot device is the use of mind control or dark magic spells on Hermione. This forces her into the slave role. Another is the idea of Bellatrix using Hermione to get to Harry or the other members of the Order. Sometimes, there's a focus on Hermione's inner strength as she tries to break free from the slavery, which is also a plot device to create tension and drama.
A frequent theme is the exploration of power. Bellatrix is a powerful and feared witch, and Hermione is also very intelligent and magically capable. Their relationship in these fanfictions often involves power struggles, sharing of power, or a combination of both. Also, there's often the theme of transformation. Both characters may change due to their relationship, for example, Bellatrix might become less cruel or Hermione might become more assertive in the face of danger. There's also the idea of secrecy, as they would need to keep their relationship hidden from most of the wizarding community.
One of the main themes is transformation. Hermione, who is usually a strong - willed and independent character, is transformed into a 'pet' in this fanfiction concept. This shows how characters can be changed by extreme circumstances. There's also the theme of captivity and freedom. Hermione is in a form of captivity, and the fanfictions may explore her attempts to gain freedom or how she copes with the lack of it. And of course, the theme of the relationship between good and evil is also present, as Bellatrix represents evil and Hermione is a symbol of good in the original series.
Redemption is also a theme. There are fanfictions where Bellatrix has a chance at redemption through her relationship with Hermione. Maybe Hermione sees something in Bellatrix that others don't and tries to bring out the good in her. It's an interesting way to explore the character of Bellatrix beyond her villainous role in the original books.
One common theme is the power struggle. Bellatrix represents the dark, powerful, and cruel side of magic, while Hermione is the intelligent and resourceful witch. Their clash creates a dynamic power struggle within the story.
I'm not sure if there is a fanfiction that meets both criteria. The concept of one character being another's slave is a difficult one to handle tastefully. There are fanfictions that delve into dark themes, but whether they can be considered well - written and not offensive with this particular premise is highly subjective. It really depends on how the author approaches the topic, their writing skills, and the way they develop the characters within that strange relationship.
One common theme is power imbalance. Bellatrix has the power as a Death Eater, and Hermione is in a submissive position as a 'pet'. Another theme could be the exploration of Hermione's resilience. Despite being in such a demeaning situation, she tries to resist and find a way out.