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.
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.
Well, in some fanfictions with the 'Harry Hermione Tonks marriage contract', it's a plot device to bring together different story elements. Maybe it was set up by some magical law from the past. It can create a lot of drama. Hermione might be conflicted as she has her own feelings for Harry, and Tonks being involved adds another layer of complication. It can make for some really engaging reading.
One possible plot twist could be that the marriage law is part of a political ploy. Maybe the wizarding government wants to unite the pure - blood and Muggle - born factions through this law. Hermione, being the strong - willed character she is, initially resists but then realizes she can use this situation to advocate for better Muggle - born rights within the pure - blood society. Lucius, on the other hand, has his own motives, perhaps trying to regain some of his lost status in the wizarding world by being seen as more 'open - minded' through this marriage.