It could be that the author wants to show the power of Bellatrix in a more extreme way. Bellatrix is already a feared character, and killing Hermione would be a huge statement of her lethality.
In this kind of fanfiction, it might be a way to shock the readers. Hermione is a beloved character, and her death at the hands of Bellatrix would be a very unexpected and tragic event. It could also be used to create a new plotline where her friends, like Harry and Ron, have to deal with her loss and seek revenge.
Severus Snape could also be a character often paired with Bellatrix. They were both on Voldemort's side, but Snape had his own hidden motives. Fanfiction might explore a relationship between them, perhaps a power struggle or a secret alliance that also involves Bellatrix's relationship with Harry in some way.
Voldemort is often paired with Bellatrix in fanfiction. Their relationship in the original series was one of master and servant, but in fanfiction, it can be explored in more complex and romantic ways. Another character often paired with her is Narcissa. Their sisterly bond can be a central focus, or it can be twisted into something more sinister or complex in fan - created stories. Also, sometimes Sirius Black is paired with her in alternate universe stories, given their family connection and the contrast in their loyalties.
Luna Lovegood might be involved. She could provide some unique magical items or knowledge to Hermione that helps in the fight against Bellatrix. Also, Severus Snape could be a character in the fanfiction. His complex relationship with both the Order of the Phoenix and the Death Eaters could add an interesting layer to the story. Maybe he has some information that Hermione needs to defeat Bellatrix.
A female witch who has been on the fringes of Voldemort's inner circle could also be part of the harem. She might be drawn to Bellatrix's charisma and be in awe of her loyalty to Voldemort. She could also bring her own knowledge of certain dark spells and potions to the group.
Hermione could be included. She might be suspicious of Harry's interactions with Bellatrix and try to interfere or help, depending on the situation. Her intelligence could also be crucial in understanding any strange magic related to Bellatrix.
Well, of course, the daughter herself is a main character. She could be named something unique like Lyra. And then there might be other characters like her friends or mentors. For example, a kind old wizard who tries to guide her away from the dark path her mother was on.
In this fanfiction, characters like Hagrid could also be involved. He could befriend Naruto and introduce him to the more positive aspects of the wizarding world. And of course, there could be other Death Eaters who might either try to use Naruto or see him as a threat due to his relation to Bellatrix.
The author 'ShadowFictionist' also does a wonderful job. Their Bellatrix Lestrange fanfictions often involve complex plots that revolve around her relationship with other characters in the Harry Potter universe, such as her interactions with Severus Snape.