Well, in some 'Harry Potter and the Prophecy' fanfiction, a plot twist could be that the prophecy was not a prophecy at all but a spell cast by an ancient wizard to set in motion a series of events for a greater purpose. For example, it was to bring about a new era of magic where Muggle - borns and pure - bloods could co - exist peacefully. This would completely change the way the characters view the prophecy and their roles in it. Also, the prophecy might be tied to a different character than Harry or Voldemort, like a minor character who suddenly becomes central to the whole story.
One interesting plot twist could be that the basilisk has an unexpected alliance. Maybe it teams up with a group of outcast magical creatures to take revenge on those who have wronged them. This would completely change the dynamic as in the original, the basilisk is mainly a tool of Voldemort.
In this fanfiction, a great plot twist might be that the gamer's knowledge of game strategies is used to outwit Voldemort in a completely unexpected way. Instead of a traditional magical duel, the gamer uses their understanding of leveling up and collecting power - ups to somehow weaken Voldemort's magic. Maybe they find a way to 'hack' into his dark magic, using skills from the game world that no one in the wizarding world had thought of before.
One interesting plot twist could be that instead of Ron leaving and coming back, Hermione actually leaves the group for a while because she discovers a dark secret about her family's past that she needs to figure out on her own. This would add a whole new layer of drama to the story.
One interesting plot twist could be that Harry discovers he has the ability to communicate with the Basilisk on a deeper level than just Parseltongue. Maybe he can see through the Basilisk's eyes and use this to his advantage in uncovering secrets within Hogwarts that even Dumbledore doesn't know. Another could be that the Basilisk is not truly a villain but was being controlled by a dark force all along, and Harry has to find a way to break that control and free the Basilisk.
Another concept is the idea of prophecies being manipulated. In some fanfictions, dark wizards or witches try to change the outcome of the prophecy by interfering with the events or people involved. This can create a lot of tension and drama as the characters fight against this manipulation.
Well, one possible plot twist could be a revelation about a character's true heritage that was hidden all along. For example, a minor character might turn out to be related to one of the major wizarding families in ways no one expected.
Well, one interesting plot twist could be that Harry discovers a hidden magical creature during the holiday that has a connection to his family's past. Maybe it's a creature that was thought to be extinct but holds the key to some long - forgotten magic within the Potter family.
In some fanfictions, a great plot twist is that the 'Prince of Slytherin' is not who everyone thinks he is. He might be under a powerful charm or disguise. And when the real identity is revealed, it turns out to be someone who has been working undercover for Dumbledore all along, which shocks both the characters in the story and the readers.
One interesting plot twist could be that the truth potion is used on a seemingly minor character who then reveals a major secret about Hogwarts' founding. For example, they might disclose that there is a hidden chamber not known to the current staff, filled with ancient magic items.
In this type of fanfiction, an interesting plot twist might be that Harry, as a girl, is underestimated by many of the male characters at first. But then she uses this to her advantage. She could pretend to be meek and mild while secretly gathering information and planning strategies. Also, her transformation could lead to a change in the way Hogwarts is run. Maybe she starts a movement for more equality between male and female students, which causes a stir among the old - fashioned teachers.