Perhaps the antagonist is a group within the demigod society that wants to change the power structure. They think that by forcing Annabeth and Percy to marry, they can control the two most powerful individuals and use their influence for their own purposes. This group uses underhanded tactics to make the marriage happen, and Percy and Annabeth must fight against this organized opposition.
It could be a jealous demigod. Maybe someone who has always been envious of Percy and Annabeth's relationship and popularity. This person finds a way to manipulate some dark magic or an old rule to force them into marriage, hoping to create chaos in their lives.
It could be a new demigod who has a grudge against Percy and somehow convinces Annabeth that Percy is a threat, leading to her tying him up. So this new demigod would be the antagonist.
Maybe the main antagonist is an old enemy of Percy's who has learned about his newfound ability to hypnotize (even if it was accidental). This enemy decides to use Annabeth as a pawn by hypnotizing her, hoping to lure Percy into a trap and defeat him once and for all.
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.
It could be an ancient and powerful god or goddess who has a grudge against Percy. Maybe they see Percy as a threat to their power or plans, so they plot his death. For example, a forgotten deity who wants to regain their former glory by getting rid of Percy, who is often seen as a hero in the Percy Jackson universe.
A group of dark forces or minor gods could be the antagonists. They might believe that the union of Percy and Artemis would disrupt the balance in a way that benefits them. So, they engineer the situation where Percy is forced to marry Artemis, hoping to use the resulting chaos to their advantage. They would likely try to keep Percy and Artemis from finding a way out of the forced marriage.
Well, one possible plot could be that some powerful outside force, like an ancient god or a magical entity, forces Annabeth to marry Percy for some grand scheme. Maybe it's to unite two powerful bloodlines or to fulfill an old prophecy. And they initially resist but as they face the situation together, they start to discover new feelings for each other.
It could be Chiron, the centaur. He's always keeping an eye on the campers and would likely be concerned if he caught them up to something. He might give them a stern talking - to and some extra chores as punishment.
A group of rogue demigods could be the main antagonists. They might be jealous of the attention the daughter gets because of her parents. So, they plot to make her life difficult and try to drive a wedge between her and the other demigods.
The main antagonist might be an ancient and powerful being that has awakened. This being could see Percy and Annabeth as a potential threat to its plans of taking over the demigod world. So, it uses its influence to have them banished, thinking that without them, it would be easier to conquer. For example, a long - forgotten Titan who has found a way to break free from its prison.
It could be an old enemy of Percy and Annabeth. Maybe a monster that they thought they had defeated long ago but has come back for revenge. This time, it might target Annabeth's daughter to get back at them.