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.
Perhaps another Death Eater. Maybe she had a secret relationship with a fellow dark wizard, and they're now in a difficult situation because of the pregnancy. It would add an interesting layer of drama and secrecy.
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.
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.
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.
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.
One possible plotline could be Bellatrix's unwavering loyalty to Voldemort being tested. Maybe she discovers something about Voldemort's past that makes her question her blind devotion. For example, she could find out about some of his more cowardly actions that he has kept hidden from his Death Eaters. Another could be a scenario where Bellatrix tries to prove herself even more to Voldemort by taking on a dangerous mission that he initially doubts she can handle. This could lead to a deeper understanding between them, or it could backfire and cause a rift.
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.
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.
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.
Well, perhaps Bellatrix discovers a prophecy that involves Harry being crucial for the future of the wizarding world in a way that benefits her own interests in the long run. So, she steps in to save him. It could also be that she has a secret love for Sirius Black (Harry's godfather), and since Harry is related to Sirius, she decides to save Harry out of some twisted sense of loyalty to Sirius.
A major plot twist could be that the father of the child is a secret enemy spy. This would add a whole new layer of drama as Bellatrix's daughter has to deal with the betrayal and also protect her child from both sides. Another could be that the pregnancy is part of a magical prophecy that she was unaware of, and it has far - reaching consequences for the wizarding world.