In the fanfic, Harry could be transported to a dimension where the Goblet of Fire has the power to grant wishes. But every wish comes with a price. Harry has to be very careful about what he wishes for as it could have unforeseen consequences for him and his friends at Hogwarts.
Perhaps Harry is transported to a time in the past during the Goblet of Fire event. He could interact with the founders of Hogwarts, and they could give him advice on how to deal with the dark forces that are at play in his own time. This would add a historical and magical depth to the fanfic.
The Goblet of Fire itself could potentially be the main antagonist in a sense. If it has a sentient or corrupted nature in the fanfic, it could be transporting Harry around against his will and causing chaos for its own mysterious reasons.
The fanfic might revolve around Harry's interaction with the Goblet of Fire due to a misunderstanding. He could think that the goblet is being influenced by dark magic and decides to investigate. As he messes with it, he discovers some hidden secrets about the Triwizard Tournament and its origins. This could lead to him having to make difficult choices to protect the integrity of the tournament and Hogwarts, all while dealing with the consequences of his meddling with such a powerful magical object.
One possible plot could be Harry discovers a hidden training ground within Hogwarts. He meets a mysterious old wizard there who teaches him unique spells and combat techniques specifically for the Triwizard Tournament tasks. This wizard could have some connection to the founders of Hogwarts, and his teachings give Harry an edge in the tournament.
A possible plot could be that the Goblet of Fire starts acting strangely. McGonagall notices some odd magical fluctuations around it. She enlists the help of Hermione, knowing her academic prowess. Together they try to figure out what's wrong. Meanwhile, Harry has a strange feeling about it all. Eventually, they discover that a dark wizard from the past is trying to use the Goblet to return. And they must stop it before it's too late.
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 idea could be exploring what would happen if a different student was chosen by the Goblet of Fire. Maybe a Muggle - born student who has to face extra prejudice during the Triwizard Tournament. It would add a new layer of conflict and show how the wizarding world reacts to a 'less likely' champion.
A possible plot is that Harry gets angry at the unfair treatment he receives during the tasks of the Goblet of Fire. For example, in one task, he might notice that the other champions have some advantages that he doesn't. His anger then gives him the motivation to find a way to overcome these unfair situations and still succeed in the tournament. He could use his anger to fuel his creativity and come up with unique solutions to the challenges.
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.
It could be Sirius Black. He has a lot of experience in the wizarding world and, despite being on the run, he could find a way to communicate with Harry secretly. He could teach Harry about dark magic defense, which would be very useful in the tournament.
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.