One possible plotline could be the challenges they face in a wizarding society that might not be accepting of their relationship. They would have to deal with the prejudices of their respective families and friends.
One interesting plotline could be Lucius' internal struggle. He's a Death Eater, but now he has Harry Potter, the Boy Who Lived, under his roof. He might have to hide Harry from Voldemort while also trying to figure out his own feelings towards the boy. Another could be Harry's adjustment. He's used to the Dursleys' neglect, so living with the Malfoys, with their wealth and different kind of 'care', is a whole new world for him. And then there could be the reactions of the wizarding world. The Order of the Phoenix might suspect Lucius has ulterior motives, while the Death Eaters could see Harry as a new target or a tool to use against Lucius.
One interesting plotline could be Lucius Malfoy having a change of heart due to Harry Potter's influence. Maybe he starts to question his loyalty to Voldemort after seeing Harry's courage and selflessness up close.
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.
In some 'Harry Marries Lucius' fanfictions, it might start with a completely different wizarding world setting. For example, a world where the traditional enmities between the Gryffindors and the Slytherins are not as strong. Harry and Lucius could meet under different circumstances, like at a diplomatic event. Their relationship could then be a slow - burn one, with them having to navigate through cultural differences, power struggles within the wizarding world, and their own personal demons. This could all lead to their marriage and the subsequent development of their relationship.
One possible plotline is that they are all framed for a crime they didn't commit. To clear their names, they must team up. Harry's quick thinking, Snape's ability to deceive when needed, and Lucius's connections all play a part. As they work together to find the real culprit, they form a bond of mutual trust and respect, which is the foundation of their new - found mate - ship.
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 plotline could be Hermione discovers a hidden prophecy that involves Snape and Lucius. Another is that Snape has to protect Hermione from Lucius' dark plans. Or maybe Hermione is a mediator between Snape and Lucius in a magical conflict.
One possible plotline could be that Harry discovers his unique connection to Severus and Lucius and has to deal with the secrecy and the implications within the wizarding world. There might be a lot of internal conflict as he tries to understand his feelings for both of them while also facing the judgment of others.
A potential plotline in such fanfiction is that Harry, in his dominant role, discovers that Lucius has some valuable knowledge or resources that he needs. So, he uses his power over Lucius to access those. Lucius, being submissive, is at Harry's mercy. Additionally, there could be a plotline where Harry's dominance over Lucius causes a stir in the wizarding society, with some supporting Harry's actions and others condemning them. This can lead to a lot of political and social drama within the fanfiction world.