Yes, there might be. Fanfiction is a vast world, and with the popularity of Percy and the Greek mythology concept, it's quite possible that someone has written a story where Percy is Zeus' mate. You can check popular fanfiction websites like Archive of Our Own or FanFiction.net.
Well, it could involve Percy having to deal with the power dynamics of being Zeus' mate. Maybe there are political intrigues in Olympus that Percy has to navigate. For example, other gods might be jealous or have their own agendas regarding this new relationship.
In this fanfiction, Fem Harry might come from the wizarding world but be a reincarnation of an ancient goddess. Zeus, sensing her divinity, is drawn to her. Their relationship could be filled with power struggles as Harry is used to being independent and Zeus is used to being in control. It would make for an interesting dynamic as they try to understand each other's worlds and powers.
The plot could involve Harry being a sort of chosen one in the world of the Greek gods. Zeus sees his potential and takes him under his wing. As they spend time together, their relationship evolves in an unexpected way. Harry could introduce some new concepts from his own world to Zeus, and they start to depend on each other in a way that makes others in the godly world think they are mates.
Obviously, Harry and Zeus are the main characters. Harry, who might be from the Harry Potter universe, would bring his own set of skills and personality traits. Zeus, as the king of the Greek gods, would have his power and authority. There could also be other characters from either the wizarding world or the Greek mythology that play supporting roles, like Hermione from Harry's world who could try to understand what's going on, or Athena from Greek mythology who might have an opinion on Zeus's new relationship with Harry.
In the fanfiction, Percy's relationship with Zeus may develop gradually. Zeus, being a powerful and often aloof figure, might initially be distant from Percy. But as Percy shows his courage and abilities, Zeus could start to take a closer interest. This might lead to Zeus mentoring Percy in some ways, teaching him about godly powers and responsibilities.
It could also be about power. Percy might realize that with Zeus out of the way, he can reshape the world of the gods in a more just and fair way. He could believe that Zeus's rule is corrupt and that killing him is the only way to bring about real change.
In this kind of fanfiction, Percy might challenge Zeus over issues like the unfair treatment of demigods. For example, Zeus could be overly harsh on demigods who make minor mistakes while the gods themselves get away with much worse. Percy, being a hero, would stand up for his fellow demigods.
Another possibility is that Percy discovers some long - forgotten weakness of Zeus. In the world of fanfiction, the author could create a scenario where Percy stumbles upon an ancient prophecy or a hidden secret about Zeus. This knowledge could give Percy the upper hand. He could then use his wits and his powers to exploit this weakness and defeat Zeus.
It could start with Fem Harry being magically transported to the realm of the Greek gods. She might be in a state of confusion and awe when she first arrives. Zeus, noticing this strange and powerful new being, approaches her out of curiosity.