One idea could be Percy exploring a hidden part of the paradise that has magical creatures never seen before. He could form an alliance with them to protect the paradise from an outside threat.
A plot could revolve around Percy meeting a doppelganger of himself in paradise. This doppelganger has a different personality and motives. Percy has to figure out if this other version is a friend or foe and what it means for his own existence in the paradise world.
Well, you could write a fanfic where a new type of kaiju is introduced into Kaiju Paradise. This kaiju has unique abilities that no one has seen before. The main character is a scientist or a researcher in the park who tries to study this new kaiju. Along the way, they face challenges from the park's management who don't want the public to know about this new discovery, and also from the kaiju itself which is very unpredictable.
One idea could be exploring the backstory of the main character in the 'dark paradise'. Maybe they had a difficult childhood that led them to the dark situation they are in now.
A possible plot for an Atreus and Percy fanfic could be centered around a prophecy. The prophecy states that only the combined strength of a Norse and a Greek hero can stop an ancient evil from awakening. Atreus and Percy are brought together by this prophecy. At first, they are skeptical of each other because of their different backgrounds and ways of doing things. However, as they start to face the challenges related to the prophecy, such as deciphering ancient texts and fighting off minions of the evil, they gradually learn to trust and rely on each other. In the end, they succeed in preventing the evil from rising, and their friendship becomes a legend in both their worlds.
One idea could be that Aphrodite takes an interest in Percy because of his unique charm and his ability to resist her powers at times. She might decide to test him in various ways, like sending love - stricken mortals or minor deities his way to see if he'll break. Another plot could involve Percy having to seek Aphrodite's help for a quest related to love or beauty, and in the process, they form an unexpected bond.
Perhaps a plot where Jenna has a special power that she doesn't fully understand, and Percy, who has some knowledge about it, tries to help her control it. Along the way, they face challenges from those who want to use Jenna's power for their own evil purposes. This plot can show the growth of their relationship as they face difficulties together.
One idea could be a new student arriving at Paradise High with a special ability that both helps and complicates the assassination attempts on Koro - sensei. For example, they might be able to sense his weak points more acutely but also have a personal vendetta against the students trying to kill him.
In a fanfic, perhaps Percy and his friends accidentally stumble upon Medusa's lair while on a quest. But instead of immediately attacking, Medusa offers them a deal. She says she will give them a valuable clue to complete their quest if they help her get revenge on one of the minor gods who wronged her in the past. Percy and his friends then have to decide whether to trust Medusa or not.
One idea could be Percy discovers he has a new ability related to his gills that allows him to communicate with underwater creatures in a more profound way. Maybe he can understand their ancient languages and has to use this new skill to solve an underwater mystery that threatens both the sea world and the surface world.
A great plot for a 'Percy Jackson at School' fanfic would be Percy competing in a school - wide competition that is secretly a test by the gods. The tasks in the competition could be related to the skills he has learned in his demigod adventures, like sword - fighting or outsmarting a maze - like obstacle. Also, he could befriend a mortal student who starts to suspect there is something different about Percy and his friends, and Percy has to decide whether to reveal the truth or keep the mortal in the dark.
Perhaps being reborn as a child of Poseidon but with a twist. Instead of having the typical water - based powers, you have the power to communicate with sea creatures in a way that no one else can, like understanding their emotions and memories. Or being reborn as a demigod who has lost their memory but slowly regains it as they go on quests and face challenges, and each recovered memory reveals a crucial part of their past and a new power or ability.