A powerful Titan who has somehow escaped Tartarus and is bent on revenge against the gods could be a great antagonist. This Titan might see Percy as a key figure in the new order of the gods and fates, and so wants to destroy him. Maybe it's someone like Atlas, who has a long - standing grudge against the Olympians and now wants to take it out on Percy as the champion of the fates.
A powerful sorcerer who once was a disciple of Hecate but turned to the dark side could be the main antagonist. He might be jealous of Percy being chosen as the champion and wants to steal Percy's powers for himself.
It could be an old enemy of Percy who has somehow gained power over the Styx. Maybe a resurrected villain from Percy's past who now wants to use the power of the Styx to take revenge on Percy and the gods. For example, Kronos' remnants could have found a way to manipulate the Styx's energy for their own evil purposes.
There could be a new Titan that we haven't seen before. This Titan could be more cunning and powerful than the others. Maybe he's been in hiding all this time, waiting for the right moment to emerge and lead the Titans to victory. He could manipulate Percy's friends against him and create all sorts of problems for our hero.
It could be a powerful demigod who has been corrupted by power. This demigod wants to be the sole champion of the arena and will stop at nothing to defeat Percy.
Another antagonist could be a powerful frost giant. If Percy is involved in protecting Asgardian interests related to Thor, a frost giant could be a major foe. They are strong and have their own motives for opposing Thor and his champion. They might see Percy as an easy target at first but soon realize his strength.
It could be one of Mars' old enemies, like a powerful Titan who has a grudge against Mars and sees Percy as a threat. Maybe this Titan wants to take over Mars and Percy stands in the way.
A group of powerful and corrupted demigods could also serve as antagonists. They might have been promised great power by some dark force if they can disrupt the work of the Fates. So they set out to target Percy and try to stop him from fulfilling his role as the God of Fates.
It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.
A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.
A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.