The plot might start with the announcement of the marriage law which forces Harry and Luna into a situation. Then they gradually get to know each other better in a new way. Luna's unique view of the wizarding world might influence Harry a great deal. They could also be involved in some sort of mystery related to the marriage law, like uncovering some hidden clauses or a conspiracy behind it. Their relationship would grow stronger as they face these things together.
Perhaps in a fanfiction, Harry is on a journey to find a sense of normalcy after all the chaos. Luna, being as unique as she is, shows him a different way of looking at life. They bond over their experiences with the Cursed Vaults and the fight against Voldemort. Eventually, they decide to get married and their wedding is a blend of both their personalities - full of strange creatures as decorations and the most delicious, magical food.
Well, it could involve Harry and Daphne being forced into a marriage due to some new marriage law in the wizarding world. Maybe they start off hating each other but gradually find common ground as they face various challenges together, like dealing with dark wizards or family expectations.
In this fanfiction, perhaps the marriage law is a political move. Harry, being a hero in the wizarding world, and Pansy, from a different pure - blood family, are made to marry. Their relationship might then be a struggle between their own beliefs and the expectations of the wizarding society. They could face many challenges together, like dealing with the reactions of their friends, especially Harry's friends who might not trust Pansy at first.
One common theme is the struggle against an imposed system. The marriage law forces them together, and they often have to figure out how to deal with their own feelings and desires while conforming to this new rule. Another theme is the discovery of new sides of each other. Since they might not have been that close before, they get to know different aspects of their personalities.
Obviously, Harry and Luna are the main characters. Harry, the famous wizard who defeated Voldemort, and Luna, the unique and dreamy Ravenclaw.
One popular 'Luna x Harry Marriage Law Fanfiction' is 'A New Beginning'. It explores how Luna and Harry navigate the challenges of a magically - enforced marriage law. The story beautifully portrays their individual personalities as they come to terms with their new relationship.
One possible plot twist could be that the marriage law is part of a dark plot. Maybe a powerful dark wizard has enacted it to create chaos. Harry and Sirius, instead of simply accepting it, start to investigate. They discover that the law is linked to an ancient prophecy that could bring either great good or great evil depending on how they handle their forced marriage situation.
One plot could be that Harry has to defend his ownership of the Luna fanfiction against those who want to claim it for themselves. This could lead to a series of adventures and magical battles within the fanfiction world.
The plot might involve their family backstory. Maybe their parents were involved in some secret magical research that put their family in danger. As siblings, Harry and Luna could be on a quest to uncover the truth about their family's past while also dealing with the normal Hogwarts drama like exams, Quidditch, and the threat of the Dark Arts. They could also have a special bond that allows them to communicate telepathically or something similar, which would add an interesting twist to the story.
It could be about Harry and Luna going on a secret adventure in the Forbidden Forest. Maybe they discover a new magical creature that no one else knows about.