A really cool plot twist might be that Hermione finds a way to travel back in time to change certain events that led to Voldemort's rise. She has to be extremely careful not to disrupt the whole timeline while trying to save the wizarding world. It's a complex and exciting idea that can lead to a lot of interesting scenarios in the 7th year of the story.
One interesting plot twist could be that in the AU, Neville Longbottom is the one secretly chosen for the Triwizard Tournament instead of Harry. It would completely change the dynamic as Neville has his own insecurities and strengths that would play out differently in the challenges.
One interesting plot twist could be that a minor character from the previous years turns out to be a key figure in the fight against Voldemort. For example, a Hufflepuff student who was always in the background might have some secret knowledge or power that helps Harry in his final battle.
Another interesting plot could be that Harry's connection to Voldemort causes some strange magical disruptions during the Triwizard Tournament. These disruptions seem like accidents at first, but Harry gradually realizes they are being caused by his own dark - side related powers that are being awakened in his fourth year.
Well, a cool plot twist might be that in this 4th - year AU fanfic, the Triwizard Tournament tasks are all designed by a rogue Death Eater who has infiltrated Hogwarts' staff. So the tasks are not just tests of courage and skill but also traps to try and harm the students. Also, Harry might discover that his parents were actually part of a secret society within the wizarding world that was fighting against a different dark force, not just Voldemort, which changes his whole perspective on his family and his role in the wizarding world.
A fanfiction could also be about Harry's relationship with his parents during his fourth year. Through some magical event, he gets to communicate with their spirits or memories more vividly. This could help him understand his parents better and also give him strength and guidance for the challenges he faces in that year.
A great plot twist could be that the Dementors are being controlled by someone other than the Ministry. They start attacking students more frequently and specifically target Harry because they are being used to find something that Harry unknowingly has. This would make the already dangerous situation in the third year even more intense.
One idea could be that Harry discovers a new, hidden magical ability within himself that helps him in the final battle against Voldemort. For example, the ability to communicate with the spirits of Hogwarts founders for guidance. Another could be that the relationships between the characters take a different turn. Maybe Hermione and Ron have a huge fight early on and Hermione teams up with Draco for a while out of necessity, which changes the dynamics of the whole group. Also, Neville could have an even more prominent role, leading a secret group of students who are training in a new form of magic that they hope will give them an edge in the fight.
A plot could be that Harry has premonitions about the Triwizard Tournament tasks before they happen. These premonitions are not clear at first, but as he gets closer to each task, he starts to understand them better. It adds an element of mystery and foreshadowing 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.
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.