One idea could be Percy discovers a new power related to the elements and has to learn to control it quickly to save his friends from a powerful enemy. For example, if he gains the ability to manipulate fire, he might accidentally set things on fire at first.
A great plot could be that an ancient evil is awakening and it can only be defeated by the combined powers of all the elements. Percy, as the 'God of Elements' in the fanfic, has to journey across different realms to gather the essence of each element. Along the way, he meets new allies and enemies, and has to overcome many obstacles related to the nature of each element.
Perhaps Percy is chosen by the gods to be the 'God of Elements' in a special ceremony. But this new title comes with many challenges. He has to deal with other demigods who are jealous of his new status. He also has to figure out how to balance the powers of all the elements within himself.
A cool plot could be that the God of Death has gone rogue and is starting to take souls before their time. Percy, with his friends, has to find a way to stop the god. They would have to search for ancient artifacts that can control or at least influence the God of Death's actions. They'd face many challenges, like traps in the Underworld and battles with deathly minions.
Maybe in the fanfic, Percy teams up with some unexpected allies, like a group of minor gods who also feel threatened by the more powerful gods' actions. Together, they form a plan to slay a corrupt major god, which involves traveling to different realms and facing various mythical creatures along the way. The story could explore themes of friendship and betrayal as they work towards their goal.
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 that Percy has to hide his Titan heritage from the other demigods at Camp Half - Blood. He discovers new powers related to his Titan side that are different from his godly powers. For example, he might have the ability to control certain aspects of nature in a more primal way compared to his Poseidon - related powers.
Another plot could involve Percy's friends getting accidentally transported to a different realm in the Percy Jackson universe. They have to rely on their friendship and the skills they've learned from Percy to survive and find their way back. For example, they might end up in the Underworld and have to navigate past various challenges without Percy at first.
One idea could be a mortal accidentally stumbling into Camp Half - Blood and having to adapt quickly to the world of demigods. Maybe they have some hidden power that only reveals itself in this new environment.
One idea could be to create a new quest within the game world. For example, Percy and his friends have to retrieve a powerful artifact that was stolen by a new type of monster. This could involve exploring new areas of the game, like a hidden underwater city that has never been discovered before. Another plot could be a tournament in the game where demigods from different cabins compete against each other in various challenges. This would show off their different skills and powers. A third idea is that there's a glitch in the game that's causing real - world problems in the Percy Jackson universe, and they have to figure out how to fix it before it's too late.
One plot idea could be Gaea's redemption. In the original series, she was a major villain, but in a fanfic, she could be given a chance at redemption. Maybe Percy and his friends discover that Gaea was being controlled by a more powerful and malevolent force all along, and they have to work with her to defeat this greater evil.
In a Percy Jackson Tartarus fanfic, a great plot could be centered around Tartarus corrupting the very essence of Percy's powers. He starts to lose control of his water - based abilities and instead they become tainted with the darkness of Tartarus. Percy then has to go on a journey within Tartarus to find a way to cleanse his powers, all while being hunted by the most dangerous creatures that lurk in the abyss.
A really interesting plot would be if Percy lost his memory and ended up in a mortal town. He has to figure out who he is on his own while still being hunted by some old enemies. The gods are aware of his situation but can't interfere directly because of some ancient rule. So Percy has to rely on his instincts and any small clues he can find about his past to get back to his normal self and his life at Camp Half - Blood.