Imagine if the Dementors from Harry Potter started to haunt Gotham. The people of Gotham have no idea how to deal with these soul - sucking creatures. Batman tries to fight them with his usual methods but fails. Then Harry arrives, and he has to teach Batman about the Patronus charm and how to fight Dementors. But there's a catch. The Dementors are being controlled by a new villain who has a connection to both the Potter world and Gotham, and this villain has a plan that involves using the Dementors to create chaos in both worlds.
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.
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.
One possible plot twist could be that Harry, under the influence of the dark side, forms an alliance with Bellatrix against the Order of the Phoenix. Maybe he discovers some dark secrets about his parents that lead him to question his previous beliefs and side with her.
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.
In some fanfictions, like 'The Unforeseen Consequences of Time Travel', a plot twist could be that when Harry and Hermione go back in time to save someone, they end up causing more harm than good. This completely flips the expected outcome on its head and makes for a very engaging read.