It could be a new demigod who has turned to the dark side. Jealous of Percy's popularity and Artemis' power, this demigod could try to create chaos in the high school and turn students against them.
Maybe an old enemy from the Percy Jackson series, like Kronos, has found a way to possess a student or a faculty member in the high school. This would be a great way to bring in an established threat in a new and unexpected setting.
It could be a powerful Titan who has escaped from Tartarus and blames Artemis for something in the past. So he's out to get her, and Percy has to stop him at all costs, even if it means his death.
A powerful Titan who has escaped from Tartarus could be the main antagonist. He might be seeking revenge on Artemis for her role in the previous battles against the Titans. This Titan could have new and terrifying powers that make him a formidable foe for Percy and Artemis.
A powerful Titan who has a grudge against both Percy and the Hunters of Artemis could be a great antagonist. Maybe he wants to use the power of the Hunters for his own evil purposes or destroy Percy to get revenge on the gods.
There could also be a group of rogue demigods who oppose the daughter of Artemis. They might be jealous of her powers or her connection to Artemis. They could try to sabotage her and Percy's quests and cause trouble for them at every turn.
The school principal could also be an interesting antagonist. Imagine if he or she has somehow been influenced by a dark power from the mythological world. They start implementing strict and unfair rules at the school that target the demigods specifically, and Percy has to figure out a way to break the influence without getting expelled.
A teacher at the high school could secretly be a minion of an evil god. They could be trying to undermine Percy and his friends by giving them unfair grades, getting them in trouble with the principal, and generally making their high school life miserable. This would be an interesting take as it's an unexpected source of conflict.
A good antagonist could be a powerful and vengeful centaur. Centaurs are known for their knowledge and combat skills. This centaur could have a grudge against both Percy, perhaps because he defeated some of the centaur's friends in the past, and the Hunters of Artemis because they protect the forests that the centaur wants to claim for his own evil purposes.
A great main antagonist could be a powerful Titan who has somehow escaped from Tartarus and is seeking revenge on the gods. He could target the Hunters of Artemis because they are loyal to Artemis, a goddess he has a grudge against, and Percy because of his connection to Poseidon.
The Olympian council itself could be the antagonist if they are the ones forcing the marriage for their own political gain. They might not care about Artemis' wishes or Percy's attempts to stop it, and they use their power and influence to push the marriage forward.
Perhaps an old enemy from Greek mythology who has found a way to infiltrate the school in a modern form. This could be a minor god or a powerful monster. They want to get revenge on Percy and Nico for past defeats. They might be using the school's resources, like the science labs or the library, to create chaos or find a way to regain their full power.