One interesting plot twist could be that despite being siblings, they are sorted into different houses, and this causes a rift in their relationship initially. But as the story progresses, they find common ground in their magical abilities and family secrets.
In some fanfics, a plot twist might be that Hermione discovers she is Draco's sister just as she is starting to have feelings for him romantically. This creates a huge moral and emotional dilemma for her, as it goes against what she thought she knew about her family and relationships.
One common plot twist is the revelation of a hidden family connection that makes them siblings. Another could be that they have to team up against a common enemy, like a dark wizard who is targeting their family. It might also be that Draco has to choose between his Slytherin friends and his new - found sister Hermione when there's a conflict between them.
One interesting plot twist could be when Hermione discovers her true heritage and has to face the fact that she's related to the Malfoys. It completely changes her relationship with Draco. Instead of being enemies, they have to figure out how to be siblings in a world where their families have such different values.
A common plot twist is that Draco is secretly working against his family's wishes to protect Hermione. For example, his family might be part of a dark plot, but he chooses to side with Hermione instead. This adds an element of conflict and drama to the story.
One interesting plot twist could be that Harry, who is often seen as the 'angel' figure, is actually tempted by dark magic in a way that makes him question his own goodness. This could be due to some spell or influence from a powerful dark force. Another twist might be that Draco, the 'devil' in the scenario, has a secret noble cause that he's been hiding under his mean exterior. For example, he could be trying to protect his family from a greater evil by seeming to side with the wrong side.
One interesting plot twist could be that Draco saves Hermione not out of pure altruism but because he discovers a dark plot against the wizarding world that Hermione unknowingly holds the key to solving. So, by saving her, he's also saving himself and his family from a greater threat.
One possible plot twist could be that Draco's family initially opposes the marriage. His pure - blood - supremacist parents might have a hard time accepting Hermione, a Muggle - born. Another twist could be that there are dark forces at play trying to use their union for some evil purpose, perhaps related to an ancient prophecy. And a third twist could be that Hermione discovers a long - lost family secret that has some connection to the Malfoy family, which changes the dynamics of their relationship and the marriage.
One interesting plot twist could be that they are betrothed due to a magical contract made by their ancestors to end an old feud between the Gryffindors and Slytherins. Hermione at first resists but then discovers Draco has a kinder side she never knew. Another could be that in the middle of their betrothal, a dark wizard tries to use their union for his own evil purposes, forcing them to work together to stop him. And perhaps, after all the chaos, they realize they actually love each other in a way they never expected.
A possible plot twist could be that Hermione gets turned into a Death Eater against her will, and Draco has to help her break free from the dark magic controlling her. This would completely reverse their typical roles.
Perhaps a plot twist could be that Draco is under the influence of a dark spell when he kidnaps Hermione, and as she tries to break the spell, she uncovers a conspiracy within the wizarding world. Or it could be that the kidnapping is a ruse set up by Hermione herself to infiltrate the Slytherin group, but Draco doesn't know it and things spiral out of control.
In some of these fanfictions, a great plot twist is when Hermione gets cursed and Draco has to find a way to break it. During this process, they learn more about each other's true selves. Also, it could be that Draco is under the influence of a dark spell that makes him act against Hermione, but she believes in the good in him and tries to break the spell, which deepens their bond in a very unexpected way.