One interesting plot could be them discovering their relation later in life. For example, Hermione might be adopted without knowing her true family ties. Then, through some magical mystery, she finds out she's related to Lucius. It could lead to a lot of inner turmoil for both of them as they come from very different worlds within the wizarding community.
Another plot might involve them teaming up against a common enemy. Maybe there's a dark wizard threatening the wizarding world who has a vendetta against their family line. Lucius, with his Slytherin cunning, and Hermione, with her Gryffindor bravery and intelligence, could put aside their differences and use their combined skills to defeat the foe.
A third plot could be centered around the Malfoy family's reaction to Hermione being Lucius's sister. The Malfoys are known for their pure - blood pride, and Hermione being a Muggle - born (or so they thought) would cause quite a stir. Lucius would have to navigate his loyalty to his family and his new - found sister, while Hermione would have to deal with the prejudice from the Malfoys and their associates.
Well, sometimes in these fanfictions, a common plot is that Hermione and Lucius are forced into the marriage by the law at first. But as time goes on, they start to discover unexpected similarities in their love for knowledge. For example, Hermione's thirst for magic knowledge and Lucius' in - depth understanding of the dark arts (not in an evil way but in a historical and academic sense). They might then team up to solve some magical mystery together, which not only brings them closer but also changes their relationship dynamic completely.
A third plot could be centered around their different friend groups. James is likely to be friends with more mischievous types, while Hermione has her studious friends. Their sibling relationship could be tested when their friends' actions or opinions clash. But in the end, they realize that family is more important and find a way to bring their two worlds together.
Lucius has a sense of entitlement that comes from his pure - blood upbringing and family wealth. Hermione, being a Muggle - born (in the original story), has had to fight for her place in the wizarding world. So in fanfiction as siblings, Lucius might initially look down on Hermione's more inclusive views, while Hermione would be appalled by Lucius's prejudice. However, as the story progresses, they could learn from each other. Lucius could become more open - minded and Hermione could understand the pressures of a pure - blood family.
Perhaps there is a plot where Hermione is being wrongly accused of a crime she didn't commit. Lucius and Draco, despite their past enmity, come to her support. Draco, having grown to respect Hermione's intelligence over the years, convinces his father. They use their family's resources to clear her name and expose the real culprit.
One interesting plotline could be the clash of their different ideologies. Hermione, with her strong beliefs in equality and justice, and Lucius, from a traditional pure - blood family with elitist views. Their marriage, forced by the law, might lead to a power struggle within the Malfoy household as Hermione tries to assert her values.
Another interesting plot is a time - travel one. Hermione goes back in time and has to interact with a younger Lucius and Snape. It's fascinating to see how her knowledge of the future affects her interactions with them and how they react to her. For example, she might try to prevent certain events that Lucius or Snape were involved in, which could change the course of the wizarding world.
One possible plot could be their struggle between loyalty to Voldemort and their own moral compasses. For example, Harry might be forced to do dark deeds under Voldemort's command but tries to find a way to resist subtly. Lucius, on the other hand, may be torn between his long - held pure - blood beliefs and the extreme actions Voldemort demands. They could also have interactions where they either compete for Voldemort's favor or team up for a common goal within the dark lord's regime.
Another plotline might involve a magical accident that causes Hermione to have a change in personality. This new, bolder Hermione starts flirting with Lucius out of curiosity and to see how the pure - blooded wizard will react. It could lead to some tense and interesting interactions between them.
One possible plotline could be Lucius having a change of heart due to Hermione's intelligence and strong will. Maybe he starts to question his Death Eater beliefs as he interacts with her at Grimmauld Place. Another could be Hermione using her knowledge to outwit Lucius in some sort of magical duel or intellectual battle within the house.
One idea could be a plot where Hermione, through some magical accident, is transported to a time when Lucius was young and they meet under different circumstances. Maybe she has to work with him to solve a mystery that could save the wizarding world in that era. This would force them to put aside their initial differences and form an unlikely bond.