It could be Voldemort himself. Given her extreme loyalty to him, it's not a far - fetched idea that there could be some sort of connection between them that led to her pregnancy.
One idea could be exploring how Bellatrix's pregnancy affects her loyalty to Voldemort. Maybe she starts to question some of his more extreme actions as she thinks about the future for her unborn child.
One possibility could be Rodolphus Lestrange, her husband in the wizarding world. It would be a traditional choice and could explore how their relationship evolves during the pregnancy. Another option could be Voldemort himself. It would be a very dark and twisted scenario, but it would add a lot of drama to the story.
Many fanfics explore the theme of redemption for Bellatrix. They imagine scenarios where she could have turned away from the dark side. Her complex character, with her madness and loyalty, also makes her a great subject for psychological exploration in fanfics.
A plot could focus on the relationship between Bellatrix and the father of the child. If it's a fellow Death Eater, how does their dynamic change? Do they become more protective of each other? Or perhaps the father is an unexpected character, like a wizard who was undercover among the Death Eaters. This would add a whole new layer of mystery and drama to the fanfiction.
Another theme is redemption. Many fanfics explore the idea of the daughter trying to atone for her mother's sins or break free from the cycle of evil that her mother was a part of. For example, she might try to use her magic for good instead of following in Bellatrix's footsteps.
The plot could involve the son trying to redeem his mother's name. He might go on a journey to find out more about her past, hoping to understand why she became so evil. Along the way, he meets various characters who knew his mother and has to deal with their mixed reactions towards him.
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.
There might be a fanfic titled 'A New Beginning for Bellatrix's Daughter'. In this, her pregnancy could be a symbol of hope in a rather dark and chaotic wizarding world. It could show her journey of self - discovery as she becomes a mother, breaking away from the shadow of her infamous mother.
She might be motivated by a long - lost connection. For instance, if there was a family tie between Harry and her that she was previously unaware of. Or perhaps she has a change of heart after seeing the true nature of Voldemort's actions and decides that Harry is the only hope to stop the chaos and destruction Voldemort is causing. It could also be that she is magically bound to save Harry due to some ancient spell or prophecy that she discovers.