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.
Maybe there's a plot twist where the Death Eaters are actually being controlled by an even more powerful and hidden dark force. This would turn the whole fight against the dark side on its head. The characters would have to reevaluate their strategies and alliances. It would also add a new layer of mystery and danger to the story.
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 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 great plot twist could be related to their friends. Since they are twins, their friends from Gryffindor and Slytherin are forced to interact more. Hermione and Ron might initially be very skeptical of Draco being Harry's twin. But as they get to know him better through the connection with Harry, they find that there are good sides to Draco that they had never noticed before.
One interesting plot twist could be that Snape's blood adoption gives Harry some of Snape's memories. This would allow Harry to see Snape's true motives throughout the years, not just the ones Dumbledore had revealed. Another could be that the adoption gives Harry a new magical ability related to Snape's potions expertise, like the ability to brew extremely powerful potions without much effort.
Well, when it comes to different house Harry Potter fanfiction, the plot twists are diverse. In Gryffindor fanfic, a cool plot twist could be that a Gryffindor who was always seen as a hero makes a huge mistake that they have to atone for. This shows that even the bravest can err. For Slytherin, a plot twist might be that a Slytherin forms an alliance with a group that they were initially opposed to, for a strategic reason that surprises everyone. In Hufflepuff fanfic, a plot twist could be that a Hufflepuff discovers a hidden power within themself that they use to save the day. And in Ravenclaw fanfic, a plot twist can be that a Ravenclaw's intellectual curiosity gets them into a situation where they have to use their wits in a completely new way to get out.
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.
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.
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.
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 of the most interesting plot twists could be when Draco and Harry, despite being from different backgrounds in the wizarding world, find that in the vampire coven they have to rely on each other more than ever. For example, they might be forced to team up to protect the coven from a common enemy that is after their unique magical - vampire powers combination. This could lead to a whole new level of understanding between them.