A possible plot twist in a Harry Potter third task fanfiction could be that the Triwizard Cup is actually a Horcrux. This would add a whole new level of danger and mystery to the task. Another idea is that one of the other champions has been under the Imperius Curse and is unknowingly working against the other contestants. Maybe the maze itself starts to collapse and the contestants have to use magic they've never tried before to escape while still trying to reach the cup. It would make for a very exciting and unexpected turn of events.
One plot twist could be that the maze reveals different illusions for each contestant based on their deepest fears. For example, Harry could see Voldemort killing his friends instead of the normal obstacles. Another could be that the cup at the end is a fake and leads to a more dangerous trial. Or perhaps one of the other champions teams up with a dark wizard within the maze to try and eliminate Harry.
One interesting plot twist could be that instead of rescuing Ron, Harry has to rescue a different person who was secretly important to him all along, like Neville. It changes the dynamic as Neville might have different skills or knowledge that Harry has to utilize to complete the task.
In some fanfictions, Harry might struggle with PTSD after the third task. The trauma of facing Voldemort again could haunt him, causing nightmares and making him more withdrawn. He might also become more determined to defeat Voldemort, spending all his spare time studying dark magic defenses.
Maybe Dobby could be an unexpected ally. He could use his magic to create distractions for the other contestants or help Harry find a hidden passage in the maze. Another possibility is that one of the Hogwarts ghosts, like Nearly Headless Nick, decides to assist Harry. He could float through walls and spy on the other champions' movements and relay the information to Harry.
Well, in a fanfiction of the Harry Potter first task, Harry could find out that the dragons have been enchanted by a dark wizard who wants him to fail. So, he has to not only deal with the dragons but also break the enchantment. Or perhaps Harry has a premonition about the task in his dreams, and with the help of Dumbledore, he learns how to use this new - found ability to his advantage during the task. Also, a fun idea could be that Neville gives Harry a magical plant that can temporarily paralyze the dragon, but Harry has to figure out how to use it correctly in the heat of the moment.
Perhaps a plot twist could be that the magical creatures interact with Harry differently. In the original, they had a certain way of relating to him, but in this fanfiction, being raised as a black wizard might make some creatures more or less friendly towards him. Some might see him as a symbol of change in the wizarding world and be more drawn to him, while others might be suspicious. Also, the way Harry approaches magic could be different. He might draw on cultural magic that is unique to his black heritage, which could give him an edge in certain situations or make him a target for those who fear change.
Maybe Harry's parents are alive but they've been working undercover for a long time against a different dark force that is allied with Voldemort. When they come back into Harry's life, they bring with them a whole new set of problems and mysteries. For example, they might have made some dangerous pacts with other magical beings in order to gain information. Harry has to deal with the consequences of these pacts while also trying to keep his family together. Another plot twist could be that they are alive but time - traveling is involved. They've been stuck in a different time period and Harry has to figure out how to bring them back to the present, all while Voldemort is still a threat.
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.
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.
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.