Well, perhaps in the fanfic, an accidental potion makes Blaise gain the ability to see the future for a short time. This would completely change his actions and decisions as he tries to prevent or cause certain events. Or it could be a potion that makes him invisible against his will, and he has to figure out how to reverse it while being in the middle of some important event at Hogwarts.
One interesting plot twist could be when a potion that was meant to be a simple healing draught accidentally turns into a truth - revealing potion. This might lead to characters revealing secrets they never intended to. Another could be a potion that causes temporary magical ability swaps among the characters, like Blaise and another student, which would create a lot of chaos and new interactions.
If an accidental potion causes Blaise to lose his voice, it would be a big challenge for him. He's used to being able to communicate his thoughts and opinions, especially in a place like Hogwarts where social status and being able to speak up matter. This would force him to find other ways to interact with his friends and classmates, like using magic to write messages in the air. And it could also lead to some misunderstandings, especially if others misinterpret his silence.
A really interesting plot twist would be if Harry Potter was somehow transported to Gotham as a child and raised there. He would grow up with a different set of values and skills, and when he discovers his magical abilities later on, he has to balance his newfound magic with the street - smart tactics he learned in Gotham. This would completely change his character and how he approaches the fight against Voldemort. And when Batman eventually meets him, he's not the typical Harry Potter we know from the books. It could lead to a whole new dynamic between them.
One interesting plot twist could be that Harry goes back in time and accidentally changes the sorting of the houses. This would lead to different friendships and rivalries. For example, if he made it so that Draco was sorted into Gryffindor, it would completely change their dynamic. Another could be that he changes the outcome of a Quidditch match in the past, which then has a ripple effect on the school's morale and subsequent events. Maybe Gryffindor loses a crucial match they were supposed to win, and it makes them more determined in other aspects.
Well, perhaps the plot twist is that it's all a dream sequence. Harry has been under a lot of stress with the whole Voldemort situation and his mind creates this strange scenario in Potions class. Or, it could be that Blaise has a twin who is actually a good guy and Harry mistakes him for Blaise in the heat of the moment and kisses him, leading to a whole lot of confusion and hilarity.
In certain fanfictions, a really interesting plot twist is when Harry Potter is sent back in time and ends up influencing Tom Riddle's early life in ways he didn't anticipate. This can lead to a completely different Hogwarts and wizarding world. For example, Harry might accidentally introduce a new form of magic that Tom then misuses. Or, there could be a plot twist where Tom Riddle's Horcruxes have an entirely different origin or purpose than in the original books, which throws Harry's understanding of how to defeat him into chaos.
One possible plot twist could be that Harry discovers he has a long - lost sibling. This would completely change his understanding of his family and his place in the wizarding world. It would also have a huge impact on the relationships he has with other characters, like Dumbledore who might have been hiding this truth from him.
Imagine if the Marauder's Map could show the layout of Narnia. Harry and his friends could use it to navigate through Narnia, which would be a huge advantage. But then, there could be a twist where the map starts to malfunction in Narnia because the magic there is different. And a character like Mr. Tumnus could be revealed to have some connection to the wizarding world in Harry Potter. Maybe he was once a wizard who was exiled to Narnia and lost his magic, but regains it when the two worlds start to interact.
One interesting plot twist could be that Fem Harry Potter, with her magic skills, ends up changing the power dynamics in the Highschool DxD world. For example, she might introduce new spells that disrupt the existing balance of power among the various factions. Another could be that she has a hidden connection to one of the powerful characters in Highschool DxD, like a long - lost relative or something. This would lead to a whole new set of relationships and storylines. Also, perhaps her presence causes the different species in Highschool DxD to reevaluate their views on magic, as her magic might be different from what they are used to.
Well, one plot twist could be when Lucius, instead of using the Imperius curse on Harry as expected, uses it on Draco to make him spy on Harry. And it turns out Draco is actually a double agent working with Harry in secret.
One interesting plot twist could be that the Death Eaters in the Harry Potter world form an alliance with the Britannian Empire from Code Geass. This would create a powerful and evil force that the protagonists from both worlds would have to fight against. Another could be that a character like Lelouch, with his Geass power, gets transported to Hogwarts and uses his wits and power to outmaneuver Voldemort. Maybe he could even use his Geass on some of Voldemort's followers to turn them against him.