It might be a new kind of monster created by the combination of Greek and Amazonian dark magic. This monster is immune to most normal weapons and has the strength to take on both Amazons and demigods easily. To defeat it, Percy comes up with a brilliant plan using his knowledge of both worlds, and Wonder Woman bows to him in admiration for his quick thinking and bravery in the face of this terrifying new villain.
It could be that Percy Jackson has somehow saved Wonder Woman's life in a unique and unexpected way. For example, he might have used his powers to manipulate water in a way that stopped a deadly weapon from harming her. In gratitude and recognition of his power and heroism, Wonder Woman bows to him.
The main antagonists could be the Titans who see Percy as a threat because of his combined powers. They might try to stop him from fully realizing his potential as the son of Wonder Woman.
The main villain could be an ancient Titan who was imprisoned long ago but has now found a way to break free. This Titan could be seeking revenge on the gods and demigods alike, and Percy and his friends would have to stop him before he destroys the world as they know it.
Circe is another option. She has the power to transform people and cause chaos. In the fanfiction, she could be jealous of Wonder Woman's beauty and power and Percy's connection to the gods. She might try to turn them against each other by using her magic on them or on those around them, causing misunderstandings and battles.
Kronos could make a comeback. Even though he was defeated before, he could find a way to return in a more powerful and vengeful form. He could be the mastermind behind the apocalypse, seeking to finally destroy the gods and take over the world once and for all.
Maybe a group of rogue nymphs who are angry at how the demigods have been treating the natural world. They trap Percy as a warning to the other demigods, and he has to negotiate with them and find a way to make amends for the actions of his kind.
Maybe a new type of monster that is created or awakened during the eclipse. This monster could be immune to most of the demigods' powers during the eclipse, making it a very difficult foe. It could have the ability to manipulate shadows, which are stronger during the eclipse, and use them to its advantage in battles against Percy and his friends.
Perhaps a minor god who is jealous of the attention the major gods get. This god decides to cause trouble at the zoo. He could be the one responsible for making the magical creatures in the zoo act up. He would try to stop Percy from uncovering his plans and restoring order at the zoo. He might have some strange powers related to the natural elements found in a zoo, like the ability to make plants grow wildly to entangle Percy or make the water in the ponds turn into quicksand - like substances.
It could be Kronos again, but in a more powerful and resurrected form. He might have found a way to come back even stronger after his previous defeat and now has a new plan to take over the world of gods and demigods.
Maybe a group of rogue demigods who have decided to take over the Hollow for their own purposes. They could be using the resources of the Hollow in a wrong way, like trying to extract its magic to make themselves more powerful. They would be a formidable foe as they have demigod abilities.