It could be another Titan who opposes the idea of balance. Maybe a Titan who wants to tip the scales in their favor and create chaos. This Titan could see Percy as a threat to their plans and constantly try to stop him from achieving balance.
A powerful demigod gone bad could also be the main villain. This demigod might be influenced by the idea of the Titan of Balance in a wrong way. They could believe that they can use the Titan's power to rule over both the mortal and demigod worlds. So they would be in direct conflict with Percy who is trying to maintain the proper balance.
It could involve Percy facing new challenges related to the concept of balance. Maybe he has to deal with a Titan who is disrupting the natural order. For example, the Titan might be causing chaos between the mortal and the demigod worlds, and Percy has to find a way to stop it.
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.
The main antagonists could be the Titans who are still bitter about their defeat and want to disrupt the new balance that Percy is trying to create. They might see Percy as a threat to their plans of regaining power. Another group could be some rogue demigods who have been corrupted by power and don't want Percy to enforce balance as it would limit their own abilities to do as they please.
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.
Maybe it could be a corrupted sea nymph who has made a deal with the remnants of Titan power. She was once a friend to Percy but has now turned evil due to her greed for more power. She starts causing trouble in the sea, like making whirlpools that endanger ships and sea creatures. Percy would be heartbroken to have to fight her but also determined to stop her madness.