One possible main antagonist could be Draco Malfoy. He might be jealous of Hermione getting all the attention as the champion and try to sabotage her. Another could be some of the pure - blood supremacists who don't want a Muggle - born to succeed in the Triwizard Tournament.
One interesting plotline could be Hermione facing unique challenges as a champion that are different from what Harry faced in the original Triwizard Tournament. For example, she might use her extensive knowledge in unexpected ways to overcome tasks. Another could be how her relationships with Ron and Harry are affected. Ron might initially be jealous but then come to support her wholeheartedly. And she could also form new alliances with students from other schools in the process of the tournament.
She would likely become more confident in her magical abilities. As a Triwizard Champion, she would be constantly tested and forced to rely on herself more. This would make her even more self - assured and less likely to doubt her own knowledge and skills.
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.
There are fanfictions where Hermione and Fleur become friends during the Triwizard Tournament. In some stories, they start to see past their initial rivalries. Fleur might initially see Hermione as just a book - smart girl, but as they face the challenges of the tournament together, they develop a mutual respect. Maybe they work together on the Yule Ball preparations within the context of the tournament, which helps to strengthen their bond.
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 some fanfictions, Hermione might be developed as more of a rebel. For example, she could be tired of always following the rules in the Triwizard Tournament and decides to break a few to gain an advantage. This shows a different side of her, one that's willing to take risks.
Harry and Ron would be her most likely allies. They are her best friends, and even if they might be initially shocked or jealous of her being in the tournament, they would surely come around to support her. Another likely ally could be Cedric Diggory. He's a fair - minded and kind wizard, and he might see Hermione's intelligence and skills as an asset and offer to help or at least be friendly towards her.
One common plot is Hermione being an unexpected champion. Maybe she gets chosen by the Goblet of Fire due to some magical interference or a misunderstanding. Another plot could be her facing unique challenges during the tasks. For example, in some fanfictions, she uses her vast knowledge to outwit the challenges in a different way than the original champions. Also, there are often plots where Hermione has to deal with the jealousy and suspicion of her peers, since she's not supposed to be in the tournament in the first place.
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.