A plot could involve Bellatrix in a mission that goes wrong. She might get separated from the other Death Eaters and has to survive on her own in a muggle area. This would force her to use her magic in creative ways to avoid detection and also make her face her prejudices against muggles. Or, it could be a love story where she falls for an unexpected character, which throws her whole world into chaos as she tries to balance her feelings with her loyalty to Voldemort.
One idea could be exploring Bellatrix's backstory before she became a Death Eater. Maybe she had a complex relationship with her family that led her down the dark path. Another could be a story where she has a change of heart, perhaps due to an unexpected encounter with a character like Harry Potter. For example, she could see some of her own past self in Harry and start to question her loyalty to Voldemort.
Perhaps a fanfic could be about Bellatrix's time in Azkaban. How did she survive the Dementors? What were her thoughts and dreams during those long years? Maybe she made some sort of pact with a dark force in Azkaban that enhanced her powers when she got out. This could lead to a story where she becomes even more dangerous and unpredictable, and the other Death Eaters start to fear her more than they already did.
Well, a cool plot for a 'Harry Potter and Bellatrix Fanfic' might be that Bellatrix, in a moment of confusion during a battle, uses a spell that transports her and Harry to a different time period. There, they have to rely on each other to survive and in the process, Bellatrix starts to question her loyalty to Voldemort. Also, a plot where Bellatrix discovers that Harry has a hidden power that could be used to bring back someone she lost, and she bargains with him for it could be interesting.
Well, perhaps there's a magical accident that causes Bellatrix and little Harry to be stuck in a time - loop or a magically sealed room together. At first, Bellatrix is enraged and wants to kill Harry immediately. But as time passes, she starts to notice Harry's courage and tenacity, which reminds her of the qualities she once admired in herself or others. Harry, in turn, tries to find a way to escape while also trying to figure out how to use Bellatrix's internal conflict to his advantage.
One idea could be a story where Bellatrix has a moment of doubt about Voldemort's methods. Maybe she starts to see the destruction he's causing and has to decide whether to stay loyal or turn against him. And Harry Potter could somehow be involved in this internal struggle of hers, perhaps by showing her a different path through his own actions and values.
One idea could be a time - travel plot where Hermione somehow gets sent back in time and has to deal with a younger Bellatrix. Maybe she tries to change Bellatrix's path to the dark side. Another could be a magical duel between Hermione and Bellatrix where Harry is caught in the middle, forced to choose which side to help. Or a plot where Bellatrix, under some sort of spell, has to team up with Hermione and Harry to fight a greater evil.
One plot idea could be Harry growing up in a very different environment. Instead of the Dursleys, he's raised in the dark wizarding world under the influence of Bellatrix and Voldemort. He might struggle with his identity, having the blood of both a hero (in a way, as he's related to Harry Potter in the original story) and two of the darkest wizards. He could be trained in dark magic from a young age but have an inner conflict due to some latent goodness in him.
A really interesting plot could be Bellatrix being given a chance at redemption. In this fanfiction, she could start to question Voldemort's motives after seeing the destruction he causes. She might then try to secretly help the Order of the Phoenix. Also, a plot about her teaching a young Death Eater the ways of the Dark Arts and having second thoughts about it mid - way through could be fascinating.
One idea could be a time - travel plot where Harry somehow travels back in time and has to interact with Bellatrix in her younger days. This could lead to him trying to change her path before she becomes a Death Eater.
Some fanfics explore Bellatrix's family relationships more deeply. Like her relationship with Narcissa. There could be a plot where Bellatrix has to choose between loyalty to Voldemort and protecting her sister, which adds a lot of emotional depth.
Well, it could be that Fem Harry Potter is actually a long - lost relative of Bellatrix's family. Bellatrix initially hates her because of her association with Dumbledore and the Order. But as the story progresses, they find out some dark family secrets that connect them. They then have to decide whether to keep fighting against each other or team up against a greater evil, like a new dark wizard who is more powerful than Voldemort and is threatening the wizarding world.
One plot idea could be a story where Bellatrix, in her unique form, discovers a young and powerful Female Harry Potter in a hidden magical realm. They start as adversaries but gradually come to understand each other's motives and powers, leading to an unlikely alliance against a greater dark force that threatens both their worlds.