It could be an old enemy of the gods, like Kronos. Even though he was defeated before, he could find a way to come back and target Percy and Apollo specifically. Since Percy has been a thorn in the side of many of the Titans' plans before, and Apollo is a powerful god, Kronos might see them as a threat to his new - found or potential power.
Maybe a new, powerful demigod who has been corrupted by dark magic. This demigod could be jealous of the friendship between Percy and Apollo and their combined powers. So, they try to break them apart and take over the demigod world for themselves. They would use their own powers, which could be a unique combination of different elements, to create chaos and oppose Percy and Apollo at every turn.
It could be Hades. He might not like the idea of Percy, a son of Poseidon, being friends with Apollo. Hades could see it as a threat to his own power or plans in the underworld.
Well, it could be about Percy and Apollo becoming friends in an unexpected way. Like, Apollo is in a situation where he loses his godly powers temporarily and Percy helps him out. During this time, they bond over their shared experiences of being different from regular humans. They start to understand each other's strengths and weaknesses, and form a strong friendship.
A powerful demon or dark spirit could be the main antagonist. This entity could be attracted to the combined power of Naruto and fem Apollo. It might seek to drain their powers to become even more powerful itself. For example, it could be a demon from Naruto's world that has somehow crossed over into Apollo's realm and is causing chaos there, targeting both Naruto and fem Apollo because they pose a threat to its plans.
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.
It could be a dark figure from Greek mythology who wants to stop Naruto from fulfilling Apollo's will. Maybe a Titan who is jealous of Naruto's new - found connection to Apollo.
One possible scenario could be Percy teaching Apollo and Artemis about modern mortal world. Percy could introduce them to things like skateboarding. Apollo would be excited and eager to try, while Artemis might be more reserved but still curious. They could bond over these new experiences and their shared sense of adventure.
It could be a trainer who takes advantage of Ash's loneliness. This trainer might trick Ash into giving up his Pokémon or getting into dangerous situations. For example, he could tell Ash that there's a special place where he can find friends if he follows him, but it's actually a trap.
The main antagonist could be a powerful sorcerer who has found a way to enter the Percy Jackson game world. He has his own motives, perhaps he wants to steal the powers of the demigods for himself. He's able to manipulate the game's code to his advantage, creating illusions and false paths for Percy and his friends to follow. Or it could be an ancient spirit that was sealed away in the game but has been released. This spirit is angry at the gods and demigods and wants to destroy everything they hold dear. Also, a group of bandit demigods could be the main antagonists. They don't follow the rules of the game or the real - world demigod society and are causing chaos everywhere they go, trying to steal valuable items from both the game world and the real - world locations.
Nyx could be a great main antagonist. She's a primordial goddess of the night and Tartarus is her domain in a way. She might see Percy and his friends as intruders and try to stop them from leaving or achieving their goals. She could use her power over darkness and the unknown to create all sorts of obstacles for them.
It could be a dark wizard who has infiltrated Hogwarts under a disguise. Percy might be the one to first suspect this wizard's true identity.