One idea could be that Percy encounters a sea monster with tentacles in a new, uncharted part of the ocean. The tentacles could be enchanted and try to trap him, but he uses his water powers to escape. Another plot could involve a tentacle - like creature that is actually a guardian of an ancient underwater temple that Percy needs to enter. Maybe the tentacles are made of some kind of magical energy that he has to figure out how to get through.
One idea could be that Percy has to seek Hephaestus' help to repair a powerful weapon that is key to defeating a new threat. Maybe the weapon was broken in a battle against a mysterious monster. Another plot could involve Hephaestus tasking Percy with retrieving a special metal from a dangerous location, like the heart of an active volcano, in order to create a new, even more powerful weapon for the demigods. And perhaps there's a sub - plot where Hephaestus' children are in some sort of trouble and Percy has to team up with them to solve the problem.
One idea could be Percy having a vision or encounter with Hestia that leads him to a hidden part of the Underworld where the warmth of Hestia's hearth is the only thing keeping a powerful and dangerous entity at bay. He has to work with Hestia to keep it that way.
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.
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.
One idea could be that Hylla and Percy team up to rescue a group of demigods captured by an ancient monster. They use their combined powers and wits to outsmart the beast.