Obviously, the gods themselves would be the main antagonists. Since they are the ones who kill Percy and Jason in this concept. It could be a group of gods, like Zeus, Poseidon, and Hera, who feel that the two heroes are getting too out of hand.
It could also be an ancient and forgotten god who has been awakened. This god could manipulate the other gods into believing that Percy and Jason are a danger. So, while the other gods are the direct killers, this forgotten god is the real instigator behind the whole thing. This would add an interesting layer of mystery to the fanfic.
One possibility is that the gods see Percy and Jason as threats to their power. Maybe they've become too powerful or popular among the demigods. So, the gods plot against them in secret. They could set up a series of seemingly impossible challenges that are actually traps. Eventually, Percy and Jason fall into these traps and are killed. This could lead to chaos among the demigods as they lose two of their most powerful leaders.
It could be an old enemy from their past, like a resurrected Titan or Giant. These powerful foes might manipulate Percy and Jason into betraying their friends as part of a grand plan to regain power or take revenge on the demigods.
A great antagonist could be a rogue demigod who has been corrupted by power. This demigod could have discovered a way to combine the darkest parts of Greek and Roman magic and is using it to cause chaos among both camps. He or she might see Jason and Percy as the biggest threats to their plans and target them specifically.
A group of rogue demigods could be the antagonists. They are jealous of Percy and Jason's fame and power. They form a secret alliance and start causing trouble for the other demigods. Percy and Jason meet while trying to figure out who is behind these attacks. At first, they suspect each other, but then they realize the real culprits are this group of rogue demigods and join forces to stop them.
The main antagonist might be a new character, a powerful being that rules the realm where Percy is banished. This being could see Percy as a threat to his or her power and try to stop Percy from finding a way back.
The gods themselves could be the main antagonists. Since they cursed Percy, they might be trying to keep him in a state of misery or use him for their own purposes.
There's a chance that it's a new entity created by the imbalance between order and chaos. This entity is a force of pure destruction and wants to tip the scales completely in favor of chaos. Percy, Nico and Jason, with the help of Chaos, have to stop this entity from achieving its goal.
A possible main antagonist could be Eris, the goddess of strife. She might be jealous of Aphrodite's beauty and power and see Percy as a tool to disrupt Aphrodite's plans. Eris could send her minions to create chaos and conflict between Percy and Aphrodite, trying to make them turn against each other.
Nico di Angelo could be a central character. Given his connection to the underworld and his own complex relationship with the gods, he might be the one who has the most knowledge about what really happened to Percy and how to reverse it. He could be on a quest to bring Percy back from the dead, facing many challenges along the way.
It could be an envious demigod who feels that Percy doesn't deserve his new position as a consort of the gods. This demigod might plot against Percy, trying to undermine his influence among the gods and the other demigods.