Well, perhaps the betrothal was actually a mistake due to a misread prophecy. And when they find out, Harry and Blaise have already developed deep feelings for each other. So now they have to decide whether to fight against the revelation or accept that their relationship was based on a false premise and try to move on. This could lead to a lot of internal conflict and drama in the story.
One possible plot twist could be that the betrothal was a setup by a dark wizard to gain control over Harry. Maybe they thought by tying him to a Slytherin like Blaise, they could manipulate Harry more easily.
There are some. It really depends on what kind of story you're looking for. Some might focus more on the relationship development between Harry and Blaise, while others could be set in different magical scenarios. For example, some stories might be during their time at Hogwarts, and the betrothal could be part of an old wizarding family tradition that they have to navigate while dealing with the usual Hogwarts drama like classes, Quidditch, and rivalries.
In this fanfiction, a great plot twist could be that the Slytherin character has a hidden identity as a member of an ancient magic - protecting group. The betrothal to Harry is part of a bigger plan to unite the houses against a new, emerging dark force. As they come together, they face a lot of opposition from both sides, Gryffindor and Slytherin, but through their relationship, they start to bridge the gap between the two houses.
One common plot twist could be that Harry discovers a hidden prophecy related to his new status as lord betrothed. This prophecy might involve some sort of danger that he has to face not only for himself but also for his betrothed. Another could be that his betrothed is actually from an enemy family in disguise, leading to a lot of internal conflict for Harry. Also, there might be a plot twist where the magic that made him lord betrothed has some unforeseen side effects that start to surface as the story progresses.
In a Sofia and Hugo betrothed fanfiction, a plot twist could be that the neighboring kingdom, which has always been an ally, suddenly turns against them because of their betrothal. This puts Sofia and Hugo in a difficult position as they have to deal with the political fallout. Another plot twist could be that Sofia is secretly in love with someone else, a commoner perhaps, and she has to struggle with her feelings for Hugo and her true love. Or, Hugo could be under a spell that makes him forget his love for Sofia, and Sofia has to find a way to break the spell and save their betrothal.
One possible plot twist could be that Blaise Zabini's family secrets come to light during their relationship. Maybe there are dark family alliances or hidden magical artifacts in his family's history that impact their marriage. For example, an old family curse that starts to affect Harry.
A third plot twist could involve a character we least expect coming to Harry's rescue. Perhaps a Slytherin student who has been secretly against Voldemort all along. This student could have some inside knowledge about Voldemort's lair and manages to free Harry, leading to a change in the dynamic between the houses at Hogwarts.
One possible plot twist could be that Harry has hidden magical abilities passed down from Regulus that no one expected. Maybe he can communicate with certain magical creatures in a unique way.
The fanfiction could explore the political implications of Harry being betrothed to a Lestrange. It would cause a stir in the wizarding community. Some would see it as a dangerous move, while others might hope it could lead to peace. Harry would have to deal with the public's perception, as well as his own doubts. He may also uncover secrets about the Lestrange family that could change his view of the situation completely. He could be caught between loyalty to his friends who are skeptical and his newfound connection to the Lestrange he is betrothed to.
Perhaps the Black family member is in danger, and the betrothal is a way to protect her. Harry gets involved in the Black family's secrets and power struggles while trying to uphold the betrothal. Another plot could be that it was a magical accident or a prophecy that led to the betrothal, and they have to figure out how to deal with it.
A great plot twist could be that the Death Eater she's betrothed to is actually a double agent himself. All along, Hermione thought she was the one with the upper hand in the undercover operation, but it turns out he's been playing a long - game too. This would completely change the power dynamic between them and add a whole new layer of complexity to their relationship.