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.
One interesting plot twist could be that instead of Cedric Diggory being the Hogwarts champion, Neville Longbottom is chosen by the Goblet of Fire. This would completely change Neville's character arc in this part of the story, as he'd have to face much more danger and scrutiny. It could also lead to new friendships and rivalries.
One interesting plot twist could be a character who was thought to be a friend actually being in league with the Death Eaters all along. For example, a minor character like Seamus Finnigan could be secretly working for Voldemort, passing information about Harry's movements. This would add a whole new layer of danger and suspicion to the story.
A great plot twist could be that the Goblet of Fire was actually tampered with by a group of former Death Eaters who wanted to bring back Voldemort in a different way. They planned to use the tournament as a cover to perform a dark ritual, and Harry uncovers this plot halfway through the story.
One interesting plot twist could be that a different student is chosen by the Goblet of Fire instead of Harry. Maybe a Slytherin student who then has to deal with the suspicion of being in cahoots with Voldemort. Another could be that the tasks in the Triwizard Tournament are completely different, like instead of facing a dragon, they have to solve a complex magical riddle within a dangerous enchanted forest.
One interesting plot twist could be a different champion being secretly controlled by Voldemort instead of just Barty Crouch Jr. as the imposter Moody. Maybe a champion from a different school who no one suspects.
One interesting plot twist could be that Harry has an unexpected ally in the Triwizard Tournament. Maybe a Slytherin student who has a hidden agenda but ends up helping Harry in a crucial task. This would change the dynamic of the traditional House rivalries.
In this fanfic, it might be that Harry tricks the Goblet by changing the rules of the Triwizard Tournament without anyone noticing. He could have found a loophole in the magic that governs the Goblet. For instance, he figures out a way to make the Goblet consider his participation as a form of magical experiment rather than a normal entry. So, instead of being a traditional champion, he becomes a special case that the Goblet has to accept due to the new set of rules he has cleverly created.
One interesting plot twist could be that Fem Harry forms an unexpected alliance with a Slytherin student. Maybe she realizes that not all Slytherins are as bad as they seem in the context of the Triwizard Tournament. This changes the whole dynamic of the story as she now has an inside source in a house that was previously seen as her rival.
In some fanfictions, Snape could be the key to unlocking the mystery behind the Goblet of Fire's malfunction. Perhaps he discovers a dark magic spell that was cast on the Goblet long ago and has to race against time to reverse it. Along the way, he has to interact with Harry and the other students in ways that he normally wouldn't, which could lead to some very interesting character development.
A possible plot twist is that Draco is secretly helping Harry in the Goblet of Fire tasks. He might do this because he realizes that there are darker forces at play that could harm not just Harry, but also Hogwarts and his own family. So, he uses his knowledge of the dark arts in a positive way to assist Harry without anyone knowing.