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's a possibility that it's an old enemy of the Potter family who has resurfaced. They know that Harry is a symbol of hope in the wizarding world, and by forcing him into the dangerous tournament, they aim to shatter that hope. They could be using some ancient and forbidden magic to influence the Goblet, and they stay in the shadows, manipulating events from afar.
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 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.
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.
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 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.
Maybe it's a jealous classmate. Someone who is envious of Harry's fame and power. This person discovers a dark spell that forces Harry to be a girl, thinking that it will make Harry lose his status among his friends and in the wizarding community at large. They then watch from the sidelines as Harry struggles with this new identity.
A powerful dark wizard who has a grudge against those with fire - related magic could be the main antagonist. He might see Harry's newfound 'Son of Fire' powers as a threat to his own plans for domination.
One possible interesting plot could be Harry's internal struggle with being forced into the Triwizard Tournament. He might have to deal with the pressure of competing while also trying to figure out who or what is behind this forced entry. Another plot could involve how his relationships with Ron and Hermione change due to this unexpected situation. Maybe Ron becomes jealous at first because Harry gets all the attention, but then they reconcile as they realize it's not Harry's fault.
Well, it might be that the Goblet of Fire has been corrupted in the fanfiction. It could be causing chaos in the Triwizard Tournament, endangering the lives of the students more than it should. Harry, being the hero, takes it upon himself to destroy it to restore order. For example, if it starts spawning dangerous and unfair tasks that are clearly not part of the original tournament's design.