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 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.
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.
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.
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.
One possible plot twist could be that they go into Slytherin and are initially shunned by the other Slytherins. But then they discover a hidden group within Slytherin that has different values from the traditional Slytherin ones. This group is more inclusive and values knowledge and friendship over pure - blood supremacy. Harry and Hermione then work with this group to change the perception of Slytherin within Hogwarts.
One interesting plot twist could be that Harry, in joining Slytherin, discovers a long - hidden family connection to one of the prominent Slytherin families. This changes his perception of his own identity and makes him question his place in the wizarding world.
Well, it could be that Harry discovers his betrothal to a goddess through some magical means. Maybe a prophecy or an ancient magic. The goddess could be from a different magical realm and bring all sorts of new magic and challenges into Harry's life. Their relationship might start rocky as Harry is used to the Muggle - born and wizarding world rules, but gradually they learn to understand and love each other.
In some fanfictions, a great plot twist is that the 'Prince of Slytherin' is not who everyone thinks he is. He might be under a powerful charm or disguise. And when the real identity is revealed, it turns out to be someone who has been working undercover for Dumbledore all along, which shocks both the characters in the story and the readers.
The plot could involve Harry's struggle to be accepted in Slytherin while still maintaining his own identity. His court in Slytherin could be a group of outcasts in the house who he befriends and together they work against the pure - blood supremacist ideas that some Slytherins hold. This would create a lot of conflict and drama as they go against the traditional Slytherin hierarchy.
Well, at first, most of Harry's Gryffindor friends would be taken aback. They have always had a certain animosity towards Slytherin. But as time goes on, if they see that the Slytherin Harry is betrothed to is kind - hearted and doesn't hold the same prejudices as some of the other Slytherins, they might start to come around. For instance, Seamus might start to joke around with the Slytherin, and Dean could start to include them in group activities. It would be a slow process of acceptance as they get to know the Slytherin better through Harry.