One plot twist could be that the Triwizard Tournament itself is a ruse. It was never really about the competition but about luring Harry into a trap. The other champions are unknowingly part of the plan. When Harry discovers this, he has to convince the other champions to join him in fighting against the real enemy behind the tournament.
In some fanfictions, the author might explore the idea that the person who dragged Harry into the Triwizard Tournament has a personal vendetta against him. Maybe it's a relative of a Death Eater who wants to see Harry fail publicly or even die during the tournament. Harry then has to not only survive the tasks but also figure out who is behind this plot against him.
A plot twist could be that the Triwizard Cup is actually a portal to a different dimension, and Harry and Hermione accidentally get transported there during the final task. They then have to find a way back while dealing with strange creatures and magic in this new dimension.
One possibility could be that Harry discovers some dark magic or a hidden conspiracy within the Triwizard Tournament that forces him to take extreme measures and destroy it. Maybe he finds out that the tournament is being used as a cover for an evil plot against Hogwarts or the wizarding world in general.
One possible plot twist could be that Harry is actually in a magically induced coma rather than dead. Maybe a powerful dark spell was cast on him that made everyone believe he was dead, but a hidden ally discovers the truth and tries to revive him. Another twist could be that Harry faked his own death as part of a master plan to expose a great evil within the Triwizard Tournament. He could be secretly working behind the scenes to gather evidence and take down the real villains.
Maybe in the fanfiction, there's a plot twist where Hermione discovers a hidden rule in the Triwizard Tournament that allows her to form an alliance with all the champions instead of competing against them. This would change the whole dynamic of the tournament and lead to a completely different kind of story.
One interesting plot twist could be that the de - aged Harry has some hidden memories of the future that help him in the tournament in unexpected ways. For example, he might remember a particular spell that an opponent will use and be able to counter it easily.
A possible plotline is that Luna enters the Triwizard Tournament as an unexpected champion. Harry, already in the tournament, teams up with her to face the challenges. They use their combined skills, Harry's Gryffindor courage and Luna's Ravenclaw wisdom, to overcome the tasks.
One reason could be the pressure. Harry is constantly under the spotlight during the tournament, and he may not like that kind of public scrutiny. In fanfiction, this pressure could be amplified to an unbearable level for him.
One possible scenario in this fanfiction could be that Harry and Fleur form an unexpected alliance during the Triwizard Tournament tasks. They might discover they have complementary skills, like Harry's courage and Fleur's elegance and magic related to water (since she's from Beauxbatons). This alliance could lead to them facing the challenges in a new and interesting way, perhaps with Fleur using her Veela charm in a creative manner to distract the obstacles while Harry uses his Gryffindor bravery to take on the more physical parts of the tasks.
One great fanfiction could be where Harry and Luna team up in a unique way during the Triwizard Tournament tasks. Maybe Luna uses her off - beat magical knowledge to help Harry solve the puzzles in the maze. For example, she might recognize some magical creatures that Harry is unfamiliar with and guide him through.