One plot idea could be that Hades and Poseidon have to team up to face a new, powerful threat from the underworld that is starting to seep into the seas. They have to combine their powers, Hades' control over the dead and the underworld, and Poseidon's mastery of the seas. Another could be a story about their childhood, how they supported each other during the early days in Olympus when they were often overshadowed by Zeus. Maybe they shared a secret adventure that no one else knew about.
Their relationship can be portrayed as a complex one. They are brothers, so there is an underlying bond of family. However, they also rule very different realms. For example, Hades might be jealous of Poseidon's more 'visible' and vast realm of the sea. At the same time, Poseidon could be envious of Hades' power over life and death in a way. But they also rely on each other in some ways, like when there are threats that could affect both their domains.
Well, it could focus on the political implications. In Greek mythology, the gods often have power struggles. Their marriage could be a strategic move. Poseidon and Hades might join forces to gain more influence among the other gods. Maybe Zeus would be threatened by their union. This could lead to a series of events where they have to prove their loyalty to each other and their new combined rule while facing opposition from Zeus and his allies.
As 'lemon' fanfiction is not something that should be promoted, we can look at normal Hades and Poseidon fanfiction themes. These may include their part in the creation and governance of the world, and how their personalities clash or complement each other in various mythological events.
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.
Power dynamics is a common theme. Given that both Poseidon and Hades are powerful gods, how their powers interact in the context of a relationship can be explored. For example, one might write about how they use their powers to protect or impress each other.
In the development of their relationship, they would also have to deal with family matters. The other gods, like Zeus, might try to interfere. But Poseidon and Hades could stand united. They would have to communicate a lot, about their pasts, their fears, and their hopes for the future. Through these deep conversations, they would form an unbreakable bond. Their marriage would be more than just a union of two powerful gods, but a relationship based on mutual respect, love, and the shared goal of creating a new order in the mythological world.
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 popular place to find such fanfiction is Archive of Our Own (AO3). Just search for 'Poseidon and Hades love' on their site, and you'll get a list of relevant works.
One common theme is their power struggle. Since they both hold significant power in the Greek pantheon, stories often explore how they jockey for influence. Another theme is their family relations. They are brothers, and many fanfictions delve into their complex brotherly bond. For example, they might have to reconcile after a fight that endangers the stability of the cosmos.