In a fanfiction, Poseidon and Zeus could be involved in a bet. The bet could be about who can better handle the affairs of a particular mortal city for a certain period. As they interfere in the mortal world, they learn more about each other's capabilities and develop a newfound respect.
Perhaps in the fanfiction, there could be a prophecy that involves Percy, Zeus and Poseidon. The prophecy states that only when the three of them perform a certain ritual together can they prevent a great disaster. But the ritual requires them to put aside their differences and trust each other completely. Percy has to convince the two powerful gods to participate.
A cool plot could be Percy being sent on a quest that involves mending the rift between Zeus and Poseidon. Along the way, he meets old foes and new allies. He has to use his wits and the powers he got from his fathers to convince other gods and demigods to support his cause. For example, he might have to show the children of Zeus and Poseidon that they can work together instead of being rivals.
One plot idea could be a story where Poseidon and Athena have to team up to face a common threat to the pantheon. Maybe a new and powerful deity emerges who wants to overthrow the existing order. Their different powers and personalities would create interesting dynamics as they try to work together.
One possible plot could be that in a modern - day world, Zeus, Poseidon and Hades are reincarnated as ordinary humans but still retain some of their godly traits. The 'lemon' aspect might be some sort of romantic or sensual undertones in their relationships as they navigate this new world and their feelings for each other.
One plot idea could be that the OC is a young mermaid who accidentally crosses into Poseidon's territory. Poseidon, initially angry, is intrigued by her courage. Another could be the OC is a mortal with a special power to control water to a small extent, and Poseidon takes an interest in mentoring the OC. Or perhaps the OC is a demigod sent on a quest by the gods, and Poseidon has to decide whether to help or hinder the OC.
One plot idea could be Percy having to go on a quest specifically guided by Poseidon's hidden messages in the sea currents. He has to decipher these messages to find a lost artifact that can save the underwater kingdom from a new, mysterious threat.
One idea could be Percy exploring an underwater realm that only Poseidon can access. He discovers ancient secrets about his father's past and the origin of his powers. Another could be a story where Percy has to team up with Poseidon to fight a new and powerful sea - born enemy. Maybe there's a prophecy that only their combined strength can fulfill.
One idea could be that Hera and Zeus are forced to work together on a task to save Olympus. During this time, they start to see each other in a new light and their relationship becomes more complex. They might have to rely on each other's strengths and in the process, old grudges start to fade.
One plot idea could be that Fem Percy, with her unique skills and courage, is wrongly accused by Zeus of a crime she didn't commit. She then has to prove her innocence while facing various challenges set by Zeus. Another idea is that Zeus, intrigued by Fem Percy's power, tries to recruit her for a special mission, but Fem Percy has to decide if she can trust Zeus. A third plot could be that there is a prophecy involving both of them, and they must work together despite their differences to prevent a great disaster.
One plot idea could be Hermione's struggle to fit in among the other demigods. Since she's the daughter of Zeus, she might have immense power that she has to learn to control. Another could be her going on a quest to retrieve a powerful artifact that only she, as a daughter of Zeus, can handle. Maybe she has to face off against an ancient enemy of Zeus that has been awakened.