One common theme is loyalty. Bellatrix's loyalty to Voldemort and Narcissa's more complex loyalty to her family and sometimes self - preservation create interesting dynamics. For example, in many fanfictions, Narcissa will try to protect her family from Bellatrix's more extreme actions.
There's also 'Bellatrix, Narcissa and the Unseen Alliances'. This fanfic explores how the two sisters might have formed secret alliances or had different agendas during the wizarding war. It really brings out their individual personalities and how they interact with other characters from the series.
Some fanfictions have a plot where Bellatrix and Narcissa discover a long - lost magical artifact. Their search for it and what they plan to do with it once found can lead to all sorts of interesting interactions with other characters like Harry Potter. For example, they might try to use it against him or it could be a way to redeem themselves.
In some fanfictions, Bellatrix's loyalty to Voldemort is re - examined in the context of her relationship with Narcissa. For instance, Narcissa might try to persuade Bellatrix to question Voldemort's motives, which could lead to a rift in Bellatrix's unwavering devotion. This gives a new angle on Bellatrix's character, making her more complex than just a blindly loyal Death Eater.
There might be a fanfiction where Harry Potter encounters Bellatrix and Narcissa in an unexpected way. For example, they could be in a situation where they need to cooperate with Harry to solve a mystery related to an ancient magic artifact. This would force them to put aside their Death Eater - Order of Phoenix differences and create a unique plot full of tension and discovery.
There's 'Bellatrix and Narcissa: A Different Perspective'. This fanfiction focuses on a time before the main events of the 'Harry Potter' series. It shows their early years together, how they grew up in the Black family, and the seeds of their later personalities were sown. It's a great read if you want to understand their backstories better.
There are fanfictions where Harry gets captured by Bellatrix and Narcissa is caught in the middle. She has to decide whether to side with her sister or help Harry. This creates a really tense and interesting plot. For example, in some stories, Narcissa's maternal instincts towards Draco play a role in how she treats Harry.
One common theme is redemption. Sometimes Narcissa or Bellatrix are given a chance at redemption through their interactions with Harry. For example, Narcissa might start to question the Dark Lord's ways after getting to know Harry better.
One idea could be a story where Bellatrix and Narcissa are forced to work with Harry Potter due to some dark magic that binds them, and a lemon tree (represented by 'lemon' in a symbolic way perhaps) holds the key to breaking the spell. Another could be a tale set in an alternate universe where they all have to protect a magical lemon that has the power to save the wizarding world.
For Bellatrix, instead of just being a crazed Death Eater, she could be shown as having a more complex relationship with the lemon - like it reminds her of a childhood memory that softens her a bit. Narcissa could be developed to be more independent and less influenced by her family's dark ways because of her connection to the lemon. And Harry Potter could be more empathetic towards them because of the lemon's influence on their personalities.