Well, perhaps new enemies could come in the form of elemental spirits that have been awakened due to some imbalance in the world caused by Percy's increased power. For example, the spirit of fire might see Percy's enhanced water - based powers as a direct threat to its own domain. Also, there could be a secret society within the mortal world that has discovered the existence of demigods and Percy's new powers. They might try to capture him and study his powers for their own sinister purposes. And then, there could be a powerful being from another dimension that senses Percy's power and decides to invade his world to claim it as its own.
The new enemies could be the descendants of an ancient and forgotten race of sea - dwelling creatures who are jealous of Percy's growing power. They might see him as a threat to their long - lost dominion over the oceans. Another possibility is a group of rogue demigods who have been corrupted by dark magic and want to steal Percy's new powers for themselves. There could also be a powerful sorcerer from a far - off land who has heard of Percy's powers and wants to use them to break a curse that has been placed on him.
Some of the more traditional and rule - following demigod groups at Camp Half - Blood could be against him. They believe in following the established ways and Percy's rogue behavior goes against that. They might see him as a troublemaker and try to bring him back to the 'right' path.
The Titans could be potential enemies. Even though they've been defeated before, some of them might be resentful and see Percy as the Chaos Champion as a new threat to their possible plans of a comeback. Kronos, for example, might have some of his essence lingering and try to possess other powerful beings to go against Percy.
The children of Ares might see Percy as a threat. Since Cupid represents love and Ares represents war, their natures are quite opposite. The Ares kids could be jealous of Percy's powers or simply not trust him because of his different lineage.
The traditional enemies of the demigods, like the Titans or the Giants, could still be his enemies. They might see him as an abomination because of his father being Charon. Also, some of the more puritanical demigods who follow the Olympian gods strictly might oppose him.
The Titans could still be a threat. Even though they were mostly defeated, there could be some remnants or new Titans emerging. For example, a Titan who was in hiding and now sees Percy as a threat to his plans of taking over again.
There could be a time - keeper character. This person or being would be an expert in all things related to time and would either help or hinder Percy in his journey with his new power. Another new character could be a time - lost hero from the past that Percy accidentally brings forward in time.
A powerful new demigod who is jealous of Percy's secret power could be the main antagonist. This demigod might think that Percy doesn't deserve such a power and tries to steal it for himself. He could be a son or daughter of a minor god who feels overlooked and wants to use Percy's power to gain recognition.
It could be a corrupted demigod who wants the Power of Dominion for himself. This demigod might have been overlooked for too long and feels that with this power, he can finally get the recognition he deserves, but in a very wrong way.
She could be the daughter of a minor god that hasn't been introduced much in the original Percy Jackson series. For instance, the daughter of Hecate. This would give her powers related to magic, which would be really cool in the context of the story. She could use her magic to help Percy and his friends in battles or to solve some of the mysteries they encounter.
There could be a demigod who is the child of a minor god that we haven't seen much in the original series. For example, the child of Hebe, the goddess of youth. This character could bring a fresh perspective with their powers related to rejuvenation and vitality.