In a Triwizard fanfiction, you could write about a situation where the Triwizard Tournament is held in a different location other than Hogwarts. Maybe it's held in an ancient wizarding ruin that holds many secrets and dark magic. The champions have to face not only the regular tasks but also the hidden dangers of the new location. Additionally, a plot where the relationships between the champions from different schools develop in unexpected ways, like a romantic relationship between a Beauxbatons champion and a Hogwarts student, could add a new dimension to the story.
One idea could be a story where a new, unexpected champion enters the Triwizard Tournament. Maybe a student from a different magical school that we haven't heard of before. This new champion could bring unique magic and challenges to the tournament, causing all the other champions to have to adapt their strategies.
One interesting plot idea could be a new task added to the Triwizard Tournament that involves solving a mystery in the Forbidden Forest. Another could be a friendship forming between Harry and a Triwizard competitor from a different school that changes the whole dynamic of the tournament. And perhaps a plot where magic from the different schools combines in an unexpected way during the tournament tasks.
Another plot could revolve around a spell gone wrong during the Triwizard Tournament tasks. For example, the spell that creates the maze for the final task malfunctions and the maze becomes a living, sentient entity that changes and adapts to the champions' movements, making it much more difficult and dangerous to navigate.
One plot idea could be that during the Triwizard Tournament, Harry and Mirajane are paired for one of the tasks. They have to work together to overcome some magical creatures. Harry's quick thinking and Mirajane's unique magic skills combine to make them a formidable team.
Perhaps in the fanfiction, Fleur could be in danger during one of the Triwizard Tournament tasks. Harry, being the heroic type, decides to risk everything to save her. This act of heroism not only changes their relationship but also how the other champions and the judges view Harry. It could also lead to some changes in the overall dynamic of the tournament, like new rules being implemented to prevent such dangerous situations in the future.
A plot could be that the Triwizard Tournament tasks are magically altered to target Harry specifically, and Hermione has to use her skills to figure out who is behind it. Maybe a new dark magic is involved that she has to research. She then has to train Harry in new ways to face the challenges. For example, teaching him some ancient spells that are not commonly known.
One interesting storyline could be a deeper exploration of the relationship between the Triwizard champions outside of the tournament tasks. For example, how they might support or secretly envy each other. Another could be a fanfiction where the tasks are altered in some creative way, like a new magical creature being introduced in the first task. And some might focus on the backstories of the champions, like what Cedric Diggory was really thinking and feeling during the whole process.
One idea could be that Hermione has to deal with a new, unexpected magical challenge as a Triwizard Champion that the original tournament didn't have. For example, she might face a magical creature that can change its form and she has to use her extensive knowledge to figure out how to defeat it.
Harry could use his dark elf abilities like shadow manipulation to his advantage in the tournament tasks. For instance, in the maze task, he could blend into the shadows and avoid the various magical creatures lurking there.
A really cool plot twist in Harry Potter fanfic regarding the Triwizard Champions could be that the champions from different schools end up switching places. So the Hogwarts champion has to compete for Beauxbatons and vice versa. This would lead to a whole new set of challenges and interactions. They would have to adapt to different magical cultures and teaching styles while still competing in the tournament. Also, it could be that one of the champions is actually an animagus and uses their animal form to gain an advantage in the tasks in a very unexpected way.