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.
It could be Voldemort. He might be using the Goblet's summons to get Harry into a vulnerable position during the Triwizard Tournament so that he can finally kill him.
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.
It could be someone who wants to see Harry fail in the Triwizard Tournament. Maybe a Slytherin student who is jealous of Harry's fame. They might have found a way to manipulate the Goblet of Fire to put Harry in a difficult situation, which in turn makes Harry snap.
Maybe it's a new character, a powerful dark wizard who has a vendetta against Dumbledore. By forcing Harry into the tournament, they hope to draw Dumbledore out and make him vulnerable. This new antagonist could have some dark magic at their disposal that allows them to control the Goblet's selection process.
There is also a possibility that the main antagonist is the Goblet itself if it has been corrupted by some ancient dark magic. It could be summoning Harry not for a normal Tournament but to sacrifice him or use his power for some evil purpose, making the Goblet the force that Harry has to fight against.
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.
Maybe it's a character who was previously thought to be good but has been corrupted. They use the Goblet's summoning of Harry to create chaos in the wizarding world. For example, a teacher at Hogwarts who has secretly turned to the dark side. They could manipulate the events surrounding Harry's summoning to turn the students and faculty against each other, all while trying to achieve their own sinister goals.
Harry is the central figure here. But other characters like Cedric Diggory could be involved. Since Cedric is a Triwizard Champion, if Harry messes with the goblet, it could directly or indirectly affect Cedric. Also, the teachers like Professor McGonagall might play a role. She would be concerned about Harry's actions and try to keep Hogwarts in order. There could also be appearances from the other schools' representatives as they are all connected to the Goblet of Fire and the Triwizard Tournament.
There could also be some new characters introduced in this fanfic. Maybe a mysterious wizard or witch who has some connection to the Goblet's origin. This character could either be an obstacle for Harry in his attempt to trick the Goblet or a secret helper. Another possibility is a magical creature keeper who has some insights into the Goblet's magic that Harry needs to use in his plan.
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.