Well, Harry could be the black sheep of the Malfoy family. He might not fit in with their traditional values. For example, he could be more accepting of Muggle - borns like Hermione. This would create a rift within the family. Draco might be torn between his loyalty to his family's views and his growing bond with his younger brother Harry.
Another aspect could be the relationship between Harry and Draco's parents. Lucius and Narcissa might have different expectations for Harry compared to Draco. They could try to mold Harry into a proper Malfoy, but Harry would fight against it. This could lead to many family dramas. And at Hogwarts, Harry's status as a Malfoy could make his school life very different. He might have to deal with students treating him differently because of his family name, and he'd have to find a way to be true to himself.
Perhaps in this fanfiction, Harry's presence in the Malfoy family could change the course of some major events. For instance, Voldemort might view Harry differently. He could try to use Harry's connection to the Malfoys to his advantage, but Harry would resist. Also, Harry could introduce new ideas to the Malfoy family, like the importance of friendship across different bloodlines, which could gradually change their attitudes over time.
In this fanfiction, Harry could have a very different personality. Growing up as a Malfoy, he might be more reserved and proud at first. However, his inherent goodness from the original story would still shine through. There could be interesting dynamics between him and Draco. They might have a love - hate relationship where they compete but also secretly care for each other. Harry could also use his position in the Malfoy family to try and change some of their more negative attitudes towards non - pure - blood wizards.
This fanfiction concept is really interesting. Harry being Draco's younger brother would mean a different upbringing for Harry. He'd have access to the Malfoy's wealth and status. However, his personality might still shine through. He might be a Slytherin with a heart of gold, trying to change the family's views from the inside. His relationship with Dumbledore would also be different as Dumbledore might see him as more of a 'project' to save from the Malfoy influence.
Draco could also change. Having Harry as his brother might make him more protective or more competitive. He might also start to see the flaws in his family's beliefs. As for other characters, Ron and Hermione might view Harry differently and their friendship could be strained or strengthened in new ways.
During his Hogwarts years, Harry would have a very different experience. He'd be in Slytherin, so he'd be part of a different social circle. He'd probably have to deal with the expectations of his family to uphold the Slytherin reputation. However, he might secretly study Gryffindor - like qualities and try to incorporate them into his life, which could cause a lot of internal struggle.
One possible plot is that Draco Malfoy discovers he has a non - pureblood ancestor through some ancient family records. This shatters his view of his family's pureblood lineage. He then struggles with whether to keep this a secret or reveal it, fearing the reaction from his family and the pureblood society at Hogwarts.
One possible plot could be a moment of unexpected vulnerability for Voldemort. Maybe Draco, in a moment of courage or desperation, hugs Voldemort to show him a different side of humanity, which then makes Voldemort question his own actions.
One possible plot could be that they are in arranged relationships with other people, but they start to have feelings for each other secretly. And then they have to hide their attraction while being unfaithful to their partners.
One possible plot could be that Draco uses his French skills to charm a French exchange student at Hogwarts. He might show off his knowledge of the language, French cuisine, and culture to impress the student.
One reason could be love. Draco might have fallen deeply in love with Harry and his possessiveness is a way of showing his strong feelings. He doesn't want to lose Harry to anyone else.
Well, loyalty is a big theme. They have to figure out where their loyalties lie, whether it's to their newfound family bond or to their old friends. Also, redemption is common. Draco, in particular, often has a chance at redemption as he is now related to Harry, the hero. And there's also the theme of acceptance, accepting this new relationship and the changes it brings to their lives.