One possible plot twist could be that Narcissa uses her influence as Harry's godmother to protect him from the darker forces within the wizarding world in a more public way than in the original series. For example, she might openly oppose Voldemort's regime to save Harry, which would put her in a very dangerous position as she was once part of the Death Eater families.
Well, it would definitely change things. They might start off still being rivals, but gradually they would be forced to get along. Since Harry is Narcissa's godson, Draco would have to at least tolerate Harry's presence more than in the original story.
One great 'Harry Potter Narcissa fanfiction' is 'The Redemption of Narcissa Malfoy'. It shows a different side of Narcissa, exploring her inner thoughts and her journey after the war. It's a very character - driven story that gives Narcissa more depth than what we see in the original books.
I'm not sure if there is a specific fanfiction with such a plot. Fanfiction covers a wide range of themes and ideas, but this one seems rather unorthodox and might not be very common. There are many fanfiction platforms out there, but it's difficult to say for sure without a thorough search.
One possible story could be where Harry Potter, through some magical twist, finds himself in an unexpected situation with Narcissa in a harem - like setting. Maybe it starts with a spell gone wrong during one of his adventures that transports him to a hidden realm where Narcissa is part of a group that he has some sort of connection with. The story could then explore their relationship, how Harry adapts to this new and strange situation, and the dynamics within the harem.
One reason could be the large fan base of Harry Potter. Fans are more likely to write about him in relation to other characters like Narcissa. Since Harry has so many different aspects to his character, such as his courage and his mysterious connection to Voldemort, it gives a lot of room for creativity when paired with Narcissa. He can be the hero who changes Narcissa's view, or he could be in a conflict with her, which makes for engaging fanfiction.
Family loyalty is also a big theme. Narcissa is deeply tied to her family, and fanfictions often play with the conflict between her loyalty to her family (like Lucius and Draco) and her possible newfound understanding or connection with Harry. It creates a lot of internal and external drama.
In many Harry Potter fanfictions, Narcissa is often portrayed as a complex character. Sometimes she is shown as a mother who is desperate to protect her son, Draco. This is in line with her actions in the original series where she lied to Voldemort about Harry being dead to save Draco.
One good fem Harry Potter Narcissa fanfic could be 'The Hidden Bond'. It explores a different dynamic between a female Harry Potter and Narcissa. In this fic, their relationship starts off as a mystery but gradually unfolds into a complex bond that affects the entire wizarding world around them.
Draco Malfoy is often a very popular character in these fanfictions. Since Narcissa is his mother, his relationship with her and how it is affected by her interactions with Harry can be a major plot point.