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.
A fanfiction could be about a competition among Zeus, Poseidon and Hades, and Percy has to be the judge. The competition could be something like who can best help a small, struggling mortal town without using their full power. This would show a different side of the gods and also test Percy's ability to be fair and objective.
One great fanfiction idea could be a story where Zeus, Poseidon and Hades have to team up to face a common threat that is even more powerful than them. Maybe a primordial being awakens and they must put aside their differences. In this story, Percy could be the mediator between them, using his unique position as the son of Poseidon to try and keep the peace among the three powerful gods.
One idea could be Percy having to face a new threat underwater that only Poseidon can help him understand. Maybe a lost civilization that Poseidon once had a connection with is now causing trouble. Another could be Percy going on a quest to find a powerful artifact that was once Poseidon's but has been stolen, and Poseidon gives him special powers or guidance along the way.
One idea could be Percy going on a quest that involves uncovering a long - lost secret about his connection to Poseidon that even the gods don't know. Maybe it's a hidden power that only surfaces when he faces a certain kind of threat. Another plot could be Percy having to deal with the jealousy of other demigods because of his special status as Poseidon's son. They might try to undermine him during a crucial mission, adding lots of drama and conflict.
Maybe in the fanfiction, Zeus has left a hidden prophecy about his legacy that Percy discovers. This prophecy could involve a great power that is locked away and can only be accessed by following a series of dangerous quests. Percy then has to navigate through various challenges, including dealing with other gods who are either jealous or afraid of what this legacy might bring.
A great plot could involve Percy discovering a hidden power within him due to his relation to Zeus. He then has to learn to control it while also dealing with the envy of other demigods. For example, he might accidentally cause some chaos in the camp which makes others suspicious and jealous of his new - found abilities.
One idea could be her going on a quest to find a lost trident piece that was stolen by a powerful new enemy. This would involve facing various mythical creatures and using her unique powers as Poseidon's daughter.
One idea could be Percy as a baby having small adventures in the underwater world of Poseidon. Maybe he befriends a baby sea monster. Another could be Poseidon trying to protect baby Percy from the other gods who are suspicious of this half - god baby.
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.