A great fanfiction idea could be that Hermione enters the Triwizard Tournament instead of Cedric. Harry then becomes her biggest supporter. They train together, and Hermione uses her intelligence to come up with unique strategies for each task. During the Yule Ball, instead of Ron being jealous, he helps them prepare for the upcoming tasks, and they all grow closer as friends through the whole experience.
One possible fanfiction could be about Harry and Hermione teaming up secretly during the Triwizard Tournament tasks. They might use their combined knowledge and skills to outwit the other competitors. For example, in the maze task, they could have a special code to communicate and find their way to the Triwizard Cup faster than anyone else.
In a fanfiction, Harry and Hermione could face a lot of challenges during the Triwizard Tournament that are not in the original story. Maybe there's a dark magic spell cast on the tournament by a rogue wizard, and they have to figure out how to break it while still competing. They would rely on their friendship and loyalty to each other to get through these tough situations.