Well, perhaps the God of Thunder has lost his memory and Percy has to help him regain it while also protecting him from those who would take advantage of his weakened state. This could lead to some interesting character development for both Percy and the God of Thunder. Percy could learn more about the godly side of things from the Thunder God, and the Thunder God could gain a new perspective on demigods through Percy.
One idea could be that Percy is framed as an assassin by an ancient evil force. He has to clear his name while also fighting off real assassins sent to kill him. Another plot could involve Percy being secretly trained as an assassin by a long - forgotten god who wants him to take out a new, powerful and evil demigod. Maybe Percy discovers an assassin - like power within himself, like the ability to move silently and undetected through shadows, which he has to master to defeat a group of villains who are planning to take over Camp Half - Blood.
One plot idea could be Percy having to deal with political intrigue in Atlantis. Since he's the king, different factions might try to influence him or even overthrow him. Another could be Percy using his powers to explore uncharted areas of Atlantis and discover ancient secrets. Maybe he finds a powerful artifact that has the potential to either save or destroy Atlantis.
In a Percy Jackson gym class fanfiction, the plot could revolve around a special competition in gym class. Percy is paired with some of his least favorite classmates. At first, they don't get along, but as the competition progresses, they start to see Percy's value. Meanwhile, Percy discovers that there's a magical object hidden somewhere in the gym that a rogue demigod is after. He has to balance his team responsibilities in the competition and protecting the object, all while trying not to get in trouble with the gym teacher.
One idea could be Percy discovering a new, ancient power within himself that is even stronger than his existing abilities. Maybe it's related to a long - forgotten god or a powerful magic that was sealed away. Another plot could involve Percy teaming up with other demigods to face an all - powerful enemy that no single demigod could defeat alone. For example, a primordial being that is awakening and threatening to destroy the world of the gods and mortals.
Well, perhaps Percy gets involved in a plot where a powerful enemy from the past, like an old Roman monster that was thought to be defeated, resurfaces. As a senator, he has to use his political influence as well as his combat skills to gather an army and resources to fight this threat. This would involve making deals with other senators, some of whom may have their own hidden agendas, and Percy has to navigate through all this while still being true to his demigod nature.
A great plot could involve the water dragon being under the control of an evil sea god. Percy and his friends need to break the control spell on the dragon first. To do this, they have to find a magical conch shell that has the power to counter the spell. Along the way, they face many challenges from sea monsters sent by the evil god to stop them from reaching the dragon.
Perhaps in the fanfiction, the Elder Gods start to interfere with the modern world of demigods more directly. Percy and his friends notice strange phenomena, like the weather going haywire and magic behaving erratically. They soon find out it's because an Elder God is trying to regain its former power and is using the chaos to break through the barriers that keep it in a weakened state. Percy then has to find a way to stop it by using his unique connection to water and the help of his father, Poseidon.
One idea could be Percy having to face a new type of demon that is immune to water, his usual weapon. So he has to find a new way to fight. Another could be Percy being tricked into serving the God of Demons for a while and then realizing his mistake and trying to break free.
One idea could be exploring a new adventure where Percy has to team up with a minor god that has been forgotten. They could go on a quest to regain the forgotten god's power and status in the pantheon.
Perhaps Percy, as the God of Wisdom, has to deal with a situation where his wisdom is being questioned by the other gods. They don't believe that a former hero like him can handle the responsibilities of wisdom. So he has to prove himself by solving a complex problem that has been plaguing Olympus for centuries. He uses his unique perspective as a demigod - turned - god to come up with a solution that no one else could think of.