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.
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 a dark wizard who wants to disrupt the new relationship between the wizarding world and the queen. This wizard might see Harry's position as a threat to his own plans of taking over the Muggle world starting from the monarchy.
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 new dark wizard who is jealous of Harry's ability to lift Excalibur. This wizard might want the sword for himself to gain ultimate power. He could start causing chaos in the wizarding world, trying to draw Harry out and take the sword from him. He might even team up with some of the Death Eaters who are also interested in the power the sword could bring.
Maybe the main antagonist is a dark force that has been slowly taking over the alliances from within. Harry notices this and decides to cancel the alliances to prevent the dark force from spreading further. This dark force could be something like an ancient evil that has awakened and is using the alliances as a way to gain a foothold in the wizarding world.
A power - hungry noble could be the main antagonist. This person might be jealous of the Queen's position and wants to overthrow her, and sees Harry as an obstacle in his way.
It could be a dark wizard who wants to use the mutant for his own evil plans. He might see the mutant as a powerful tool to gain control over the wizarding world.
It could be a new player on the team who is jealous of Harry's skills and popularity. This new player might spread lies about Harry, turning the other team members against him. So the new player would be the main antagonist in this scenario.
It could be a new dark wizard who has been secretly searching for the Death Stone. Maybe someone who was in the shadows during Voldemort's time and now sees an opportunity to rise with the power of this stone.
It could be a new dark wizard we haven't seen in the original series. Maybe someone who has a vendetta against Hogwarts and manipulates Harry into doing the dirty work of destruction.