Perhaps a plot twist could be related to memory modification. One of them has their memory modified in the past which affects their relationship in the present. For instance, Tonks might have had her memory modified by some dark magic and she forgets her previous relationship with Harry and Hermione. As they try to restore her memory, they discover secrets about their past together and it changes the course of their marriage.
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.
Well, a plot twist might be that the Marriage Law is actually part of a dark wizard's plan. Hermione and Charlie then find themselves in the middle of a much larger conspiracy. They might have to use their unique skills - Hermione's intelligence and Charlie's dragon - handling knowledge - to expose the plot and in the process, their relationship could either strengthen or face challenges. Also, perhaps there are ancient magic rules related to such marriages that they need to figure out.
One possible plot twist could be that at first, Hermione and Severus despise each other due to their past differences. For example, Severus has always been rather cold towards Gryffindors, and Hermione is a proud Gryffindor. But as they are forced to spend time together because of the arranged marriage, they start to discover each other's hidden depths. Hermione might find Severus's intelligence and his complex past with Lily fascinating. And Severus could come to respect Hermione's quick wit and thirst for knowledge. Another plot twist could be that there is some external force trying to break up the arranged marriage. Maybe some dark wizards see it as a threat to their plans, or there are those in the wizarding world who oppose the union of a Muggle - born like Hermione and a former Death Eater like Severus.
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 possible plot development could be that the characters initially resist the contract. Harry might be in love with someone else, like Ginny. Hermione could be focused on her studies and not ready for such a commitment. Tonks might be angry about being forced into this situation. But as the story progresses, they start to see the benefits of working together because of the contract.
One possible plot twist could be that the marriage law forces them together initially, but they discover a deep, unexpected connection. For example, Sirius might initially be resistant as he sees Hermione as a young friend, but through shared experiences in the wizarding world, like fighting against dark forces together, they grow closer.
Maybe Harry has to train Tonks in some new form of magic that he has mastered. So he becomes her master in that sense.
Perhaps Snape and Hermione were secretly working on a time - traveling potion project. Due to some unforeseen consequences of time manipulation, Hermione ended up pregnant. It could also be that a powerful magical artifact they were studying had some sort of fertility - enhancing effect that they didn't anticipate at first.
A fanfic might explore how Tonks' Metamorphmagus ability plays a role in their relationship. Maybe she changes her appearance to surprise Harry and Hermione on special occasions. The three of them might have to face some opposition from the more traditional wizarding families, but they stand strong. Their marriage could be a symbol of a new, more inclusive era in the wizarding world. They would travel together, sharing their knowledge of different magical cultures, and write books about their adventures.
One possible plot twist could be that Snape initially resists the marriage law but then gradually falls in love with Hermione as they are forced to spend time together. He might start to see her in a new light, beyond just being a student. Another twist could be that Hermione uses her intelligence to find a loophole in the marriage law to get out of the marriage, but then realizes she actually has feelings for Snape and decides to stay.