Another plot could be a time - travel fanfiction. James Potter somehow travels to a time when Narcissa is young and in a difficult situation. He decides to help her, and in the process, they form an unexpected bond. This bond then has consequences for the future as James returns to his own time.
Well, a plot could involve Harry being raised by Narcissa Black in a world where the wizarding world is more divided than ever. Harry, influenced by Narcissa, has a different view on blood purity. He could be trained in dark arts defenses in a more Slytherin - like way. This would make his battles against Voldemort very different. He might also have a closer relationship with Draco from the start, and they could end up as allies rather than rivals. Plus, Narcissa's influence could lead Harry to have a more nuanced understanding of the Death Eaters, which would add a lot of complexity to the story.
A plot could revolve around a time - travel accident. James Potter is sent back in time and ends up in Bellatrix's era. Bellatrix, being intrigued by this strange wizard from the future, decides not to turn him in immediately. Instead, they start to learn from each other, with James trying to understand the roots of the dark side she represents and Bellatrix getting a glimpse of a different kind of magic and values.
A plot could revolve around Narcissa and Sirius in their Hogwarts days. They could be involved in a mystery within the school, like a series of strange disappearances of students. Since they are both from the Black family, they might have some inside knowledge that helps them solve the mystery, and in the process, form an unexpected bond.
One challenge is getting their personalities right. James is known to be brave, confident and a bit of a prankster, while Narcissa is from a more traditional and often dark - aligned family. So, it's difficult to make their interactions believable.
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.
One idea could be that Harry discovers Narcissa has a secret mission to undermine Voldemort from within. Maybe she's been secretly helping the Order of the Phoenix all along and Harry stumbles upon this knowledge.
One plot idea could be that Narcissa teaches Hermione about the hidden secrets of the pure - blood families in the wizarding world. This could involve ancient spells, family heirlooms, and the unspoken rules among the old wizarding families. Another idea is that Narcissa, seeing Hermione's thirst for knowledge, decides to teach her some advanced potions techniques that are not taught at Hogwarts, perhaps ones passed down in the Malfoy family.
Well, a plot might involve Harry - now James - trying to change the fate of his family. He could focus on protecting Lily in a different way than James did in the original story. Maybe he'd use his knowledge of magic from his past life to create new spells or defenses. Also, his relationship with Snape could be very different. Since he was Harry before, he might understand Snape's feelings better and try to make amends or change the animosity between them.
One plot idea could be that Fem Harry Potter travels back in time and meets James Potter. They form an unexpected friendship and she has to navigate the differences between their time periods while also dealing with her own identity in this new situation.
One idea could be that they return with new magical abilities. Maybe James has the power to control time briefly, which he uses to prevent some small tragedies in the wizarding world. Another could be that they return to find that Harry has changed the wizarding world in ways they never expected, and they have to adapt to this new world order.