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.
Well, perhaps a plot twist could be that the discovery of Hermione being Draco's sister is part of a bigger plot by a dark wizard. This wizard has been manipulating events all along to use their new - found connection for some evil purpose. Maybe he wants to use Hermione's intelligence and Draco's family influence to take over the wizarding world. So they have to work together to stop this plot and in the process, come to terms with their new relationship.
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.
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 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.
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.
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 that Hermione and Harry discover their relation much later in their Hogwarts years. Maybe they've always had a special bond but just attributed it to their friendship. Another could be that their shared heritage gives them unique magical abilities that no one else has, like a combined spell -casting power that can defeat the darkest of dark wizards.
A really interesting twist could be that Hermione's presence in Middle - earth is actually a key part of an ancient prophecy that no one knew about. As Aragorn's sister, she might be the one to unlock some great power or knowledge that is crucial for the victory over evil. This would make her a central figure in the story, not just as Aragorn's sister but as a key player in the fate of Middle - earth.
One interesting plot twist could be that despite their initial animosity, Hermione and Draco find they have very similar parenting styles when it comes to discipline. They both believe in strict but fair rules, which surprises everyone around them. Another could be that in the parenting class, they are paired for a project that forces them to spend a lot of time together outside of class, and during this time, they discover a mutual respect for each other's intelligence.
One interesting plot twist could be that when they go back in time, they find out that their families were actually friends in the past. This completely changes their perception of each other. Another could be that they accidentally change a major event in the past, which has unforeseen consequences for the present wizarding world. For example, they might prevent a certain spell from being created, which then alters the balance of power between different wizarding factions.