Well, a plot could be that Percy discovers an ancient prophecy related to Hephaestus. This prophecy says that only with the combined efforts of Percy and Hephaestus can a great evil be stopped. Along the way, they face challenges like deciphering ancient riddles, fighting off hordes of mythical creatures sent by the evil force. Also, Hephaestus might be reluctant to work with Percy at first because of some past misunderstandings between the demigods and the god. But as they progress, they learn to trust each other and fulfill the prophecy.
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 plot idea could be Percy having to use Hephaestus' skills to repair a powerful, ancient weapon that is the key to saving the demigod world from a new threat. Another could be Percy competing in a Hephaestus - inspired crafting contest among the demigods to prove his worth as the champion.
One idea could be that in the fanfiction, Hephaestus discovers Aphrodite has a special interest in Percy Jackson. Hephaestus, being the god of the forge, could create some sort of magical item that ties Percy to their story. Maybe it's a charm that makes Percy visible to the gods' love affairs in a way he wasn't before, and he has to navigate the complex relationship between Hephaestus and Aphrodite while also dealing with his own demigod duties.
One plot idea could be that Percy Jackson gets caught in the middle of a feud between Aphrodite and Hephaestus. Maybe Hephaestus asks Percy to retrieve something that Aphrodite has stolen from him, which leads Percy on a wild adventure filled with Aphrodite's love - related traps and Hephaestus' mechanical defenses. Another idea is that Aphrodite decides to use Percy as a pawn in her game to make Hephaestus jealous, creating all sorts of romantic chaos for Percy. Or perhaps Percy discovers a secret pact between Aphrodite and Hephaestus that has far - reaching consequences for the demigod world, and he has to figure out how to deal with it.
One idea could be Percy discovers a long - lost Hephaestus - made weapon that has special powers. It could be a sword that can cut through any material, but it has a mind of its own and Percy has to figure out how to control it while also dealing with other monsters that are after the weapon.
One idea could be Percy having to learn the unique skills of Hephaestus' cabin, like forging magical weapons. He might struggle at first because he's used to his own powers. Another could be him having to deal with the other Hephaestus kids who are initially jealous of his fame as Percy Jackson but then come to accept him. And he could go on a quest where his new skills as an adopted son of Hephaestus are crucial in defeating a powerful monster.
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 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.