Well, a common plot twist is the idea of soulmates. In some fanfictions, it is discovered that Hermione and Sirius are soulmates across different timelines or dimensions. This leads to their kiss as they are drawn to each other on a deeper level. Another plot twist could be a misinterpretation of prophecies. The fanfiction might have a prophecy that seems to point to Hermione and Sirius being together, which causes them to explore their relationship and end up kissing. Also, the idea of a parallel universe where the rules of the original story are different can lead to them kissing, such as a universe where age differences don't matter as much in relationships.
One common plot twist is that Sirius is actually under a spell that makes him fall for Hermione, and she has to figure out how to break it while dealing with her own feelings. Another could be that they discover a prophecy that links their relationship to the fate of the wizarding world.
A plot twist could be that Hermione is initially oblivious to Sirius' feelings. She's so focused on her studies and the fight against Voldemort that she doesn't notice the signs. But then, one day, she accidentally stumbles upon a letter Sirius was writing about his feelings for her. This throws her into a state of confusion as she never expected such a thing from him.
One common theme is time - bending. Since they are from different generations, often the stories play with time travel or alternate timelines to make their relationship possible. Another theme could be the clash and then the melding of their personalities.
In some fanfics, a plot twist could be that Severus was actually protecting Hermione all along under Dumbledore's secret orders, and Sirius discovers this and has to re - evaluate his hatred for Severus. This completely changes the dynamic between the three.
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.
Another plot twist might be that the Beast is not actually a 'beast' in the traditional sense. In the fanfiction, he could be a wizard under a false appearance curse, and Hermione discovers this. It changes the whole dynamic of their relationship because now they are both magic - users, and she has to figure out how to break the false - appearance curse rather than the typical Beauty and the Beast curse.
Often, a plot twist might involve Hermione uncovering a secret that Lucius has been hiding. This could be related to some dark magic he's involved in or a family secret. This discovery could then give Hermione some leverage over Lucius. Or perhaps there's a twist where other characters from the Harry Potter universe get involved. Like Harry and Ron coming to rescue Hermione, but she refuses to leave because she's in the middle of a complex plan to bring down Lucius from the inside.
A common plot twist is that Hermione actually has a hidden power or ability that she reveals during her captivity. For example, she might be able to use a type of magic that Draco and his associates didn't expect, which gives her an upper hand in the situation. This could lead to a change in their relationship dynamic, from captor - captive to something more like equals.
One common plot twist could be that the blackmailer is someone she least suspects, like a fellow Hogwarts student who has a hidden agenda. For example, they might be jealous of her academic achievements and use some secret they've uncovered to try and undermine her. Another could be that the blackmail involves a dangerous magical secret she's been entrusted with, and if it gets out, it could put the wizarding world in jeopardy.
One common plot twist is that Hermione discovers she's the Chosen One later than expected. For example, she might think she's just an ordinary, albeit brilliant, witch until some event reveals her true destiny. Another twist could be that the prophecy about the Chosen One was misinterpreted all along, and it was actually Hermione, not someone else. This can lead to a lot of drama and re - evaluation of the wizarding world's beliefs.