Well, a plot might involve Bellatrix being involved in some sort of forbidden magic experiment at Hogwarts. She could be trying to impress the Dark Lord even as a student. Or it could be about her trying to undermine Dumbledore's authority at Hogwarts. For example, she might spread rumors or cause small acts of chaos around the school to show her disdain for the more light - hearted and inclusive Hogwarts environment.
One possible plotline could be that Bellatrix is somehow brainwashed or redeemed early on. Harry, seeing the potential for good in her, tries to help her further. Over time, they develop feelings for each other and end up getting married. Another could be a time - travel scenario where Harry goes back in time and changes Bellatrix's path before she becomes a full - fledged Death Eater, leading to their relationship and marriage.
Perhaps she discovers a hidden power within herself that neither of her parents had. This power could put her in a unique position in the wizarding world. She might have to decide whether to use it for good or be influenced by her mother's dark side. For example, if she has a special gift for mind - reading, she could use it to help people or manipulate them like Bellatrix would.
One possible plotline could be that Pansy is in a secret relationship with someone at Hogwarts, and the pregnancy is a huge shock to her and her partner. They then have to deal with the consequences, like how to hide it from other students and teachers, and what their future might hold.
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.
One possible plotline could be Bellatrix trying to mold Harry into a dark wizard. She might use her knowledge of the Dark Arts to teach him, but Harry resists. Another could be that Bellatrix starts to have a change of heart while raising Harry and begins to question her loyalty to Voldemort. Maybe she sees something in Harry that reminds her of a better part of herself.
One possible plotline could be that Harry discovers a hidden magical world outside of Hogwarts. Maybe he stumbles upon a group of rogue wizards who teach him their own brand of magic, away from the traditional Hogwarts curriculum.
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.
Perhaps the plot could involve a time - travel related issue. Maybe someone has messed with time in Hogwarts, and Hermione's memories are the only way to figure out what went wrong. So, the Hogwarts community watches her memories to see if there are any hints about the time - travel event and how to fix it. It could also be that a new student is having trouble fitting in, and Hermione's memories of her own early days at Hogwarts are shown to help the new student feel more at home.
Well, a plotline might revolve around the new relationships that form. For example, Neville could start a herbology club and meet a new love interest who is also passionate about plants. Also, the wizarding world might see some political changes. Maybe there would be a new Ministry of Magic with more inclusive policies after the dark times of Voldemort's regime.
Well, Narcissa Black (her sister) would surely interact with Bellatrix. They could be shown having sisterly moments, but also Bellatrix influencing Narcissa towards darker ways. And then there's Sirius Black. Even though they are on opposite sides, there could be some tense family - related interactions at Hogwarts. Maybe Sirius tries to convince Bellatrix to turn away from the dark side, but she refuses and they have a big argument in one of the Hogwarts corridors.