It could be that Lucius is under the orders of Voldemort. Voldemort wants Harry alive but in his control, so he sends Lucius to kidnap Harry. Lucius might then keep Harry in some secret location, perhaps an old Malfoy manor cellar, guarded by dark magic spells. Harry would then have to find a way to escape, maybe by using his own unique magic abilities or with the help of his friends who eventually figure out where he is.
One possible title could be 'The Kidnapping of Harry by Lucius: A Fanfiction'.
One possible plot could be that Fleur has been influenced by some dark magic or a powerful enemy. Maybe she's being blackmailed to kidnap Harry. She might take him to a hidden location, perhaps an old, abandoned castle. Then, the story could revolve around Harry trying to figure out why Fleur did this and how to escape while also dealing with Fleur's internal struggle about what she's done.
One possible plot could be that Harry grows up in the Malfoy family. He might be treated differently from Draco, as he has different values. This could lead to internal family conflicts. Harry could try to use his influence within the family to change their dark views. Maybe he secretly helps the Order of the Phoenix while still being part of the Malfoy family.
One possible reason could be Sirius believing he can protect Harry better on his own. Maybe he thinks the Dursleys are too dangerous for Harry and that the wizarding world isn't doing enough. So, he decides to take matters into his own hands and kidnap Harry to keep him safe.
One possible plot could be that Harry is under the influence of a dark spell or some mind - controlling magic that makes him kidnap Ginny. Maybe he's been hexed by a powerful dark wizard who wants to use Ginny as a bargaining chip against the Order of Phoenix. So, Harry, without his true self in control, takes Ginny to a hidden location.
One possible plot could be that Harry discovers some sort of ancient magic or prophecy that links him to Voldemort and Lucius in a way that makes them mates. This could lead to a struggle within Harry as he tries to come to terms with his new identity and the expectations that come with it.
The plot might start with Harry having strange dreams or feelings that lead him to realize his connection to Lucius and Severus. As they come together, they find that they have to hide their relationship from the Death Eaters (in Lucius' case) and Dumbledore's side (initially, as it's so unexpected). They could also be on a quest to find out the origin of this bond, which might be related to some ancient magic. During this journey, they learn about each other's pasts, like Severus' love for Harry's mother and Lucius' family history, and how it all ties into their present relationship.
Well, it could start with some sort of magical encounter that binds them in an unexpected way. Maybe Harry gets caught in a spell that makes him subservient to Lucius. Then Lucius has to deal with this new situation and his own feelings towards Harry.
One possible plot could be that Harry, for some reason, has to take on the role of a healer for Lucius. Maybe Lucius gets seriously injured during a battle or some dark magic backfires on him. Harry, with his knowledge of magic and his good heart, decides to help Lucius despite their past differences. This could lead to a series of interactions where they both learn more about each other and perhaps even form an unlikely alliance.
In a fanfiction with this premise, perhaps Lucius discovers a way to control Harry using some ancient and dark magic. Harry is then made to serve Lucius in his Malfoy Manor. The plot could involve Harry learning about Lucius' secrets and using that knowledge against him. There could be a sub - plot about Harry's friends trying to rescue him. As Harry endures the hardships of being a slave, he also discovers new aspects of his magic that he can use to fight back. This would be a very dark and angsty plot compared to the original Harry Potter stories.