A plot could involve Bellatrix having a change of heart after being magically bound to Harry in some sort of spell gone wrong. They then have to work together while she battles her inner demons and her loyalty to Voldemort.
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.
Perhaps in a world where the war is over and Harry is on a mission to rehabilitate former Death Eaters. Bellatrix, who was thought to be dead, is found alive but amnesiac. Harry has to deal with her past actions while also helping her start anew.
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.
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.
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.
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 that Harry discovers his true parentage and has to struggle with his identity. He might be torn between the side of good that he has known all his life and the dark legacy of Bellatrix. Another plot could involve him facing discrimination from both the wizarding world that hates Bellatrix and the Death Eaters who might see him as a tool or a threat. For example, he could try to use his knowledge of Bellatrix's magic, which might be more complex and dark - tinged, to fight against Voldemort in a different way than in the original story.
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 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.
One idea could be that Harry discovers a hidden side of Bellatrix where she has a group of followers who are all intrigued by Harry's power. They form an uneasy alliance at first, but as time goes on, they start to respect and care for each other in their own strange way.
One idea could be a story where Bellatrix and Narcissa are forced to work with Harry Potter due to some dark magic that binds them, and a lemon tree (represented by 'lemon' in a symbolic way perhaps) holds the key to breaking the spell. Another could be a tale set in an alternate universe where they all have to protect a magical lemon that has the power to save the wizarding world.