One interesting plot twist could be that Hermione discovers a hidden power within herself that has a connection to the Chamber of Secrets. Maybe she can communicate with the basilisk in a unique way, which completely changes how she approaches the mystery.
In some 'hermione granger and the chamber of secrets fanfic', a great plot twist could be that the Chamber of Secrets is not actually a place of evil as it seems in the original story. Hermione discovers that it was originally created as a place of protection, but over time its purpose has been misconstrued. She might find ancient spells or artifacts within the chamber that prove this, and she has to convince the others, including the teachers at Hogwarts, that they need to change their view of the chamber and how to deal with it.
One interesting plot twist could be that Hermione discovers a hidden spell within the Goblet of Fire that no one else knew about. This spell could change the nature of the Triwizard Tournament in unexpected ways.
One interesting plot twist could be that Hermione discovers she's the Heir of Stone not through traditional means but during a dangerous encounter with a dark wizard. The wizard tries to use her for some evil purpose related to the power of the Stone, but she manages to turn the tables on him.
One interesting plot twist could be that Ginny was secretly under a more complex spell than in the original story, which was cast by a hidden dark wizard in Hogwarts. This could lead to a whole new set of adventures for Harry to figure out how to break it.
Another plot twist might be that the relationship between Hermione and someone unexpected, like a Slytherin student, changes drastically. In the fanfic, they could start off as enemies but then find out they have a common goal related to the secrets of the Half - Blood Prince. This could lead to an unlikely alliance and a whole new set of adventures for Hermione.
One memorable new character could be a young wizard who befriends Hermione early in the fanfic. He has some unique magical abilities, like being able to sense the presence of dark magic from a distance. His friendship with Hermione helps her in her investigation of the Chamber of Secrets.
In some fanfictions, a plot twist might be that Tom Riddle had an older sibling who also had a connection to the Chamber of Secrets. This sibling could be trying to either stop or help Harry, adding a whole new layer of mystery and family drama to the story. Well, it really changes the dynamic as it makes the whole mystery of the Chamber not just about Voldemort's past but also about a part of his family that was previously unknown.
One plot idea could be Ron and Hermione getting trapped in a part of the Chamber of Secrets that was previously undiscovered. They have to rely on each other's skills, with Ron using his strategic thinking in wizard chess to figure out the puzzles and Hermione using her vast knowledge of spells and potions to fend off any threats. Another could be a fanfiction where they go back in time within the Chamber, perhaps to the time when Salazar Slytherin was alive and they have to change the course of events to prevent future disasters related to the Chamber.
One idea could be that during detention, Hermione discovers a hidden magical artifact in the classroom. This leads to an adventure where she has to figure out its purpose and keep it safe from those who might misuse it.
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.
A cool plot could involve a time - travel aspect where a character goes back to when the Chamber was first created. They could then try to change things to prevent the current danger. Also, a fanfic could focus on the life of a minor character like Moaning Myrtle and how she copes with the new threat in the Chamber.