It could be a new god or goddess who is jealous of the attention Percy gets. Maybe they try to disrupt the conversation between Athena and Percy to cause chaos.
A powerful Titan who has escaped from Tartarus might be the antagonist. This Titan could be after Percy because he knows Percy is a key figure in the demigod world. So, during the conversation between Athena and Percy, the Titan could launch an attack, trying to take Percy out while he's distracted. And Athena would have to use her wisdom and power to protect Percy and fend off the Titan.
Perhaps an evil demigod, one who has turned against the Olympians. This demigod might overhear the conversation between Athena and Percy and use the information against them. They could try to frame Percy for something or use the knowledge Athena imparts to Percy to their own advantage, causing trouble for both of them.
One possible plot could be that Athena is giving Percy some strategic advice for an upcoming battle. Maybe she's warning him about a new type of monster he'll face and how to defeat it using his wits.
It could be a rogue ninja who has the power to corrupt the trees. This ninja wants to use the power of the forest for evil, like creating a dark chakra source from the trees' life force. Naruto has to talk to the trees to figure out how to stop this ninja.
Another possibility is the Volturi. Their strict rules and power - hungry nature could be a threat in the background. Jasper might be warning Bella about the Volturi's watchful eyes and how they view her relationship with the Cullens. He could be explaining how their actions or decisions could impact not only Bella but the entire Cullen family.
It could be a dark wizard who wants to use the dragons for evil purposes. He might try to control them and make them attack Hogwarts or other wizarding communities.
It could be a new character, like a powerful wizard who has a grudge against Percy for some reason. Maybe Percy accidentally disrupted one of his spells, and the wizard is out to get him, which makes Percy mad.
Perhaps a jealous sea nymph. She has always been envious of Amphitrite's position and power. When she sees Amphitrite bonding with Percy, she decides to be the antagonist. She spreads false rumors about Percy among the sea creatures, causing some to be wary of him and Amphitrite's association with him. She might also try to sabotage any attempts at further bonding between them, like ruining a special ceremony or event they were planning to attend together.
It could be the warden of the prison. The warden might be a cruel and power - hungry person who wants to keep Percy imprisoned for his own reasons, like getting some sort of reward or power from those who put Percy there.
A powerful dark sorcerer could be the main antagonist. He might be seeking to use Percy's skills for his own evil purposes, and when Percy refuses, he becomes the target. The sorcerer would have a vast array of dark magic at his disposal, making him a formidable foe.
It could be an old enemy of Percy's, like a vengeful god or a powerful monster. They might be targeting Percy's guardian to get back at Percy.
Obviously, Percy would be one of the main characters. For the Athena Cabin, it could be Annabeth. Since they already have a pre - existing relationship, it would be interesting to see how a 'lemon' aspect could be added to their story, like exploring a more passionate side of their relationship within the context of the Athena Cabin.