In some fanfictions, this could be possible through alternate universe scenarios. Maybe a spell gone wrong alters everyone's memories and relationships, leading to Hermione and Lucius getting married. Another way could be a political alliance in a world where the war has changed the power dynamics drastically, forcing strange bedfellows to come together.
Harry would be completely shocked. He's known Hermione as his best friend and to see her with Lucius, who was once a Death Eater, would be almost unthinkable. He'd probably try to convince Hermione that she's made a huge mistake.
For Lucius, his development could be a slow process. At first, he resents being around Hermione, but as he witnesses her magic and her kind heart, he starts to question his own values. He may begin to see the error of his ways regarding blood purity. Hermione might develop more patience. Dealing with Lucius, who has such different views, makes her learn to tolerate and try to change someone rather than just oppose them.
Hermione could also use her magic skills to her advantage. She is a powerful witch, and in the Malfoy household, she may use her magic to create a space for herself where she can be true to herself. She might also use magic to subtly influence Lucius and make him more accepting of her and her beliefs.
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.
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.
In fanfiction, the relationship between Hermione and Lucius can be quite complex. Some fanfictions might explore a scenario where they are enemies forced to work together due to certain circumstances in the wizarding world. For example, they could be paired in a mission to save Hogwarts from a new dark threat. Their different backgrounds, Hermione being a Muggle - born Gryffindor and Lucius a pure - blood Slytherin, would create a lot of tension and interesting character dynamics.
Another idea is that Hermione is there by accident. She could have been exploring the Malfoy Manor (where Lucius's office is located) out of curiosity or in search of something related to Harry Potter's past. Once in the office, she discovers a hidden secret about the Malfoy family that could change the course of the wizarding war.
Fanfiction about Hermione and Lucius can be quite interesting. Writers might play with the concept of redemption for Lucius. Maybe Hermione sees a side of him that others don't. It could also be a way to explore power dynamics in a new context, with Hermione's intelligence and Lucius's former status in the wizarding world.
Well, in fanfiction, this pairing is often a shocker. It's not something you'd expect given their opposing sides in the main 'Harry Potter' plot. But it can be a fascinating exploration of character growth, especially for Lucius who was a rather villainous character.