It could be a Death Eater who has infiltrated Hogwarts and is trying to use the Tournament to harm Harry. They might manipulate the events of the Tournament to put Harry in more danger than the other champions.
Perhaps an old rival of Dumbledore who has a vendetta against Hogwarts. This antagonist sees Harry as a symbol of Hogwarts' strength and wants to break him during the Tournament to prove that Hogwarts is not as great as everyone thinks. They could be secretly influencing the judges or even modifying the Tournament tasks.
A jealous student within Hogwarts could be the main antagonist. This student might feel that Harry is getting all the attention and glory, and decides to sabotage Harry during the Tournament. They could spread false rumors about Harry, or try to disrupt his preparations for the tasks in devious ways.
It could be Draco Malfoy. Even if he's not an official champion, he might be jealous of Harry and try to sabotage him at every turn.
Well, one plot twist could be that the tasks in the Triwizard Tournament are secretly rigged against Harry by a dark wizard within Hogwarts. Another could be that Harry discovers a hidden connection between the Tournament and his parents' past that no one else knew. And perhaps, instead of the usual competitors, there are some impostors among the champions.
Well, it could be about Harry facing unique challenges in the Triwizard Tournament. Maybe he has to deal with a new dark magic threat that the other champions don't know about. And he might use some new spells he learned secretly.
A powerful mythological creature that has a grudge against the god Percy is championing for. For example, a Titan who was once defeated by that god and now sees Percy as a way to get revenge. It would be a great adversary, with its own set of powers and minions.
One possibility could be a group of dark wizards who are jealous of Hogwarts' power and reputation. They want to bring it down and Harry, as the champion, stands in their way. Another could be a former Hogwarts student who turned to the dark side and has a vendetta against the school for some reason. They target Harry specifically because he represents everything good about Hogwarts.
It could be a dark wizard who has infiltrated Hogwarts under a disguise. Percy might be the one to first suspect this wizard's true identity.
Maybe a rogue ninja who has made a pact with a dark wizard in Hogwarts could be the main antagonist. This ninja - turned - villain has combined his ninja techniques with dark magic and is causing havoc in Hogwarts. Naruto would be sent to stop him, but he'd face a tough challenge as this antagonist knows both the ways of the ninja and the secrets of Hogwarts magic.
It could be Voldemort if the story takes place during the time he was a threat. Naruto's arrival might disrupt his plans in some way, so he would see Naruto as an obstacle.
It could be Voldemort again, but in a more powerful and death - centered form. Maybe he has found a way to fully embrace the power of death and become an even greater threat. Since Harry is the champion of death, it would be a battle of two opposing forces related to death.
It could be one of Mars' old enemies, like a powerful Titan who has a grudge against Mars and sees Percy as a threat. Maybe this Titan wants to take over Mars and Percy stands in the way.