Maybe Percy discovers a long - lost power within himself that makes the other gods view him in a whole new light. This could lead to a power struggle among the gods as they try to either befriend or oppose him. For example, he might find out he can control an element that no one has ever seen before, like aether or something similar. He then has to learn how to use this power while also dealing with the politics of the godly realm.
One idea could be Percy having to face a new and powerful enemy that even the other gods are afraid of. He has to use his unique skills and connections to defeat it.
One idea could be Percy having to face a new, powerful enemy that is a threat to his newfound position as king of the gods. This enemy could be a primordial being that has been awakened. Another plot could involve Percy trying to reform the rules among the gods, which causes a lot of internal conflict as the other gods resist change. And perhaps Percy discovers a long - lost prophecy related to his kingship that changes everything he thought he knew.
Another plot could involve Percy being tricked into taking on the role of king by a devious god who wants to use him as a puppet. Percy then has to figure out how to break free from this god's control and truly claim the throne for himself and for the good of all the demigods and mortals. He could seek the help of the wise centaur Chiron who gives him advice based on ancient knowledge.
Another idea is a love story where Percy falls in love with a minor god or goddess. This could involve them having to keep their relationship a secret from the other gods because it's forbidden. The story could explore the challenges they face in their relationship and how they deal with the jealousy and interference from other deities.
One plot idea could be Fem Percy having to prove herself to the gods in a series of trials. Since she's female in this version, she might face different expectations and prejudices from the male - dominated pantheon. Another idea is a love story between Fem Percy and a god, like Apollo perhaps, and how their relationship is complicated by the power dynamics between mortals and immortals.
One idea could be Percy going on a quest to find a lost artifact that has the power to control the weather in the Olympian realm. This would put him in conflict with some of the gods who want to use it for their own purposes.
Well, perhaps Percy could use his knowledge of the gods' weaknesses to his advantage in the pranks. Like, knowing that Zeus is afraid of being overthrown, Percy could stage a fake 'demigod uprising' just to see Zeus's reaction. Or he could play a prank on Aphrodite by making all the statues in her temple look like really ugly versions of her. This would surely get a big reaction from the goddess of beauty.
One idea could be exploring the backstory of a minor god in the 'king of gods' universe. Maybe they have a secret ambition to overthrow the king of gods and the fanfiction could detail their devious plans and how they gather allies.
Perhaps Percy is chosen by the gods to be a mediator in a great council. He has to deal with the bickering and power - plays of the gods. He uses his experiences with both the mortal and divine worlds to bring some order. Along the way, he uncovers secrets about the gods' past that could change his relationship with them forever.
One idea could be Percy having to face a new set of challenges in the world of the gods as a consort. Maybe he has to deal with the politics among the deities, like trying to broker peace between feuding gods. Another could be Percy using his new position to help demigods in a more powerful way, for example, getting better training facilities for them.
One idea could be Percy having to face a new and powerful enemy that is secretly being controlled by an ancient god who has a grudge against the Olympians. He'd have to use his unique powers and the help of his friends in unexpected ways.