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.
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.
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.
Well, perhaps in this fanfic, the Goblet of Fire might summon Harry in a different way than in the original story. Maybe it has a mind of its own and specifically targets Harry for a more dangerous and secretive task that the original Triwizard Tournament didn't include.
It could be a powerful dark wizard who has infiltrated the Ministry of Magic and manipulated the Goblet of Fire. They might want Harry out of the way to further their own dark plans.
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.
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.
Well, perhaps Harry could face unexpected magical challenges. The Goblet might summon him for a task that is far more dangerous than the Triwizard Tournament in the original story.
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.
One interesting plot in this kind of fanfiction could be that the Goblet of Fire is actually under some dark magic influence. Harry, being summoned, discovers this conspiracy. He then has to not only deal with the Triwizard Tournament tasks but also find a way to break the dark magic's hold on the Goblet. This could involve him delving deeper into ancient magic lore and seeking help from the Forbidden Forest's creatures that are often overlooked in the original story.
Perhaps there's a member of the royal court who is secretly envious of Harry's power and tries to undermine him. This person could be plotting against Harry and trying to make the queen believe that he is a danger rather than an asset. They might spread false rumors or create false evidence to get rid of Harry.