A great plot twist might be that the fight is a test set up by Dumbledore's spirit. He wants to see if Harry can overcome his prejudice towards Snape. So, it's not a real fight to the death, but rather a test of character. Harry realizes this mid - fight and has to make a choice about how to proceed. This would add a whole new layer to their relationship.
One interesting plot twist could be Snape secretly protecting Harry throughout the Triwizard Tournament in a more obvious way in the fanfiction. Maybe he uses his potions skills to give Harry an edge without Harry realizing it's Snape helping him.
Perhaps in the fanfiction, Draco Malfoy turns out to be a double agent. He pretends to be on the side of the Death Eaters like his family, but in reality, he is secretly helping Harry Potter and the Order of the Phoenix. Snape discovers this and has to decide whether to expose Draco or use his secret to their advantage. This would create a lot of tension and unexpected alliances.
A really cool plot twist might be that Snape has a secret magical bond with baby Harry. This bond was formed when he saved Harry and it gives Snape the ability to sense Harry's emotions and even transfer some of his own magical power to Harry when he's in danger. It adds a whole new layer to their relationship and Snape's role as Harry's protector.
One interesting plot twist could be that Snape's 'mastery' over Harry is actually a ruse to protect him from a greater evil. Snape, in his complex way, is using this guise to keep Harry close and train him in secret arts that Dumbledore didn't have the time or the nerve to teach Harry. This would turn the whole master - slave dynamic on its head and show Snape's true loyalty to Harry and the cause against Voldemort.
One possible plot twist could be that Snape reveals a long - hidden Christmas tradition in the wizarding world that is related to Harry's family. Maybe it has something to do with his mother Lily. For example, Snape could show Harry a charm that Lily used to create a special Christmas decoration every year, and this charm has been passed down in the Potter family. Another twist could be that Snape and Harry get stuck in a magically - created Christmas - themed maze during the holidays, and they have to work together to get out. This would force them to set aside their differences and understand each other better.
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.
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.