Perhaps the twin sister has a special power or ability that Draco initially wants to use for his own gain. He tries to befriend her under false pretenses. But as he gets to know her better, he realizes she's not just a means to an end. For example, if she has the power to communicate with magical creatures, Draco, who is interested in the more elite aspects of the wizarding world, might see this as an opportunity to impress his family. However, as he spends time with her, he starts to care about her for who she is, and they develop a relationship that goes against the expectations of their respective friend groups and families.
Another plotline could be that she was cursed as a child and lost her memories. Years later, she starts to regain her memories and realizes she is a Malfoy. Draco then has to help her navigate the wizarding world that she has forgotten while also dealing with his own feelings about having a long lost sister. It could involve adventures in finding a way to break the curse completely.
A plotline could involve Draco's family trying to hide his non - pure - blood origin for generations. When Draco finds out, he could start to question his family's values and actions. He might then try to find out who his non - pure - blood ancestors were and what their story was. Maybe he forms an unlikely alliance with some non - pure - blood characters in Hogwarts as he tries to come to terms with his new identity. There could also be a plot where the discovery of his non - pure - blood status changes his relationship with Harry Potter. Instead of being rivals, they could become allies as Draco realizes the unfairness of the blood - based discrimination in the wizarding world.
Self - discovery is also very common. Growing up as a Malfoy in the shadow of her brother, the sister often has to figure out who she really is. Is she just a Malfoy by name or does she have her own unique magic and personality? Fanfictions explore this by showing her journey, like learning new spells on her own or having a different view on the wizarding war than her family.
One possible plotline could be that Harry betrays Draco in a duel. Maybe there was some sort of misunderstanding or a secret deal that Harry made with someone else that required him to turn on Draco during the duel. For example, if there was a dark wizard threatening Harry's friends and the only way to save them was to sacrifice Draco in the duel by using a forbidden spell against him.
One possible plotline could be that Harry loses Malfoy in a dangerous magical maze. Maybe they were on a joint mission for some reason, like to retrieve a powerful magical artifact. And Malfoy, being his usual self, wanders off and gets trapped in a hidden part of the maze. Harry then has to deal with his guilt of losing Malfoy while also trying to find a way to rescue him.
Perhaps there could be a plot where Naruto's twin sister gets into some kind of trouble, like being kidnapped by an enemy ninja clan. Naruto then goes on a desperate mission to rescue her. During this time, he realizes just how much she means to him. He uses all his skills and the power of his friends to get her back, and in the end, their bond becomes even stronger.
One possible plotline could be Draco trying to hide his squib status from his family. He might go to great lengths to fake being a wizard, like using magic - like tricks or relying on others to do magic for him in front of his family. Another plotline could be Draco befriending other squibs and forming a sort of underground community where they support each other and try to prove their worth in the wizarding world. Or, he could be discovered as a squib by his enemies, like Harry Potter and his friends, and instead of the usual animosity, they feel pity and try to help him come to terms with his new reality.
In fanfiction, their relationship could be one of unexpected friendship. Draco, initially a bully, might see a different side of himself when interacting with Harry Potter's sister. He could be drawn to her kindness and independence, which are traits he may not often encounter in his pure - blooded, Slytherin - centric world.
One possible plotline could be that Harry, through some magical accident or inheritance, comes to own Draco's family manor. Draco is then forced to interact with Harry on Harry's terms to regain his family's pride. Another could be that Harry wins a bet against Draco and the terms of the bet are such that Harry now has ownership over Draco's time, making Draco do tasks for Harry.
Well, in the fanfiction, they could have a very close relationship. Maybe they protect each other from the dark forces at Hogwarts. They might share secrets that no one else knows, like about their family's hidden past or their own personal fears and dreams.