A new, previously unknown mythological creature that has the ability to manipulate the super soldier serum. It wants to spread chaos and destruction, seeing the power of the super soldiers as a means to disrupt the balance between the mortal and divine realms. Percy would have to figure out how to stop this creature and its evil plans.
Maybe a powerful Titan who has somehow managed to get hold of the super soldier technology. They could use it to create an army of super - powered minions and try to take over Olympus again. This would force Percy and his friends to not only fight the Titan but also the super - powered forces.
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.
Yes, in some Percy Jackson fanfiction, he can be depicted as a super soldier. Writers often like to blend different fictional concepts, and making Percy into a super - soldier type character can add new and exciting elements to the story, like enhanced strength, speed, and combat abilities.
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.
It could be a new demigod who has a grudge against the Olympians. This demigod might be using Percy as a tool to carry out their assassination plans.
A mythological creature that has infiltrated the government agency could be the main villain. For example, a shapeshifting monster that has been causing chaos within the agency and trying to undermine its operations. Percy would have to figure out its true identity and stop it. Another option is a rogue demigod who has turned against his own kind and is working with the wrong side of the government to cause trouble for Percy and his friends.