Maybe the antagonist could be a strict teacher on the trip. This teacher doesn't like Niall and Harry's carefree attitude and is always trying to get them in trouble, like giving them extra chores or restricting their activities during the trip.
Perhaps in this fan fiction, Niall and Harry are on a school trip to a historical site. They get separated from the group and end up finding an old diary that leads them on a mystery - solving journey. They have to figure out who the diary belonged to and what secrets it holds while trying to find their way back to the group.
It could be Voldemort. He might have influenced the Dursleys in some way to harm Harry, and Severus has to save Harry from Voldemort's long - reaching plans. For example, Voldemort could have placed a curse on the Dursleys' house that would eventually harm Harry, and Severus discovers it just in time.
It could be a new Death Eater in disguise who has infiltrated Hogwarts under a false identity. They could be causing trouble by secretly spreading dark magic among the students.
It could be a super intelligent pirate who has been secretly controlling events in the One Piece world. This antagonist might be several steps ahead of everyone, creating elaborate plans to take over the Grand Line. Luffy, with his newfound intelligence, has to match wits with this foe.
The main antagonist could be a mean-spirited mole. This mole is constantly digging tunnels under the garden and causing chaos. He doesn't like Gnomeo and Juliet's love because he believes it's disrupting the natural order of the garden. He tries to separate them by causing misunderstandings between their families.
An organization that believes in using Pokemon for dark magic could be the main bad guy. They see Iris and her Pokemon as a threat to their plans, so they constantly try to stop her. They might have their own powerful, corrupted Pokemon that they use to battle Iris. They could also be stealing Pokemon from other trainers to use in their dark experiments.
A powerful ghost that has a grudge against Danny and decides to target him during the class trip. This ghost could be an old enemy who has found a way to become stronger and is now seeking revenge. For instance, it could be a ghost that Danny once defeated but managed to come back with new powers.
It could be the person who stole Harry in the first place. Maybe they are a powerful wizard who wants to keep Harry away from his real family for some self - serving reasons. They might be afraid that if Harry reunites with his true family, they will lose some kind of power or influence they have gained from keeping Harry in the dark.
It could be the society within the wizarding world that frowns upon same - sex relationships. Even though Ron's feelings are unrequited, the general attitude of the wizarding community might put pressure on Harry, and in a way, act as an antagonist. Charlie would then be rescuing Harry from this external pressure as well as Ron's unwanted romance.
Tom Riddle himself could be the main antagonist. Even in his school days, he was showing signs of his future as Voldemort. His dark thoughts and actions towards other students or his exploration of dark magic could put him in direct opposition to Harry.