Well, a great plot twist could be that Fem Harry Potter's magic accidentally merges with some of the unique powers in Highschool DxD. This might create a new type of power that no one has ever seen before, and everyone in the Highschool DxD world would be after her to either get that power or stop her from misusing it. Another plot twist could be that she gets mistaken for a prophesied figure in the Highschool DxD lore. This would put her in a very difficult position as she has to deal with the expectations and the potential danger that comes with being seen as such a figure. And then, there's the possibility that her Hogwarts friends somehow end up in the Highschool DxD world too, which would be a huge change in the story's direction.
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.
One of the most interesting plot twists could be the sharing of powers. For example, Naruto's ability to use chakra might combine with the unique demonic powers in Highschool DxD in unexpected ways. Maybe a character from Naruto's world gains a devil fruit - like ability from the Highschool DxD universe. Another could be a character from Highschool DxD learning how to use ninjutsu, which would completely change the power dynamics in their own world.
One interesting plot twist could be that Fem Harry's presence among Voldemort's ranks causes internal conflict. Some Death Eaters might be distrustful of her because she was once the 'Boy Who Lived' in a female form. Another twist could be that Fem Harry starts to influence Voldemort's ideology in a way he didn't expect. Maybe she makes him question his pure - blood supremacy by showing that her Muggle - born friends were as powerful as any pure - blood wizard. Also, it could be that her joining was part of a long - con plan to bring down Voldemort from the inside, and she gradually starts to turn his own followers against him.
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.
In some fanfics, a great plot twist is when Ichigo's Hollow powers are misinterpreted by the DxD characters as some sort of devilish power. This leads to a lot of misunderstandings and conflict at first. Another interesting plot twist can be when Ichigo somehow ends up being related to an important figure in the DxD world through some convoluted backstory.
Well, one interesting plot twist is when the Nine - Tails in Naruto is somehow linked to a powerful devil in Highschool DxD. Imagine the chaos and new power - sharing or battling that could occur. It completely flips the script on how we see both the Naruto and Highschool DxD elements. Also, there could be a plot twist where the power - leveling system in Highschool DxD is reversed for Naruto characters, making them struggle in a new way and forcing them to adapt in unexpected ways.
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.
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.