One idea could be Percy joining the Order of the Phoenix. He could bring his unique demigod powers to the fight against Voldemort. His ability to control water might be useful in various magical battles, like putting out dark magic - induced fires or creating water barriers.
Maybe there could be a crossover of characters. For example, Annabeth could team up with Hermione. They are both intelligent and resourceful. Annabeth could use her strategic thinking from the Percy Jackson world to help the Order plan their attacks, while Hermione could introduce Annabeth to new magical concepts.
Another plot could involve Percy's sword, Riptide, having some sort of connection to a powerful magical artifact in the Harry Potter universe. This connection could be the key to defeating Voldemort. Percy and his friends would have to embark on a quest to figure out this connection and use it to their advantage.
Well, a cool plot could be that the Order of the Phoenix mistakes Percy for a dark wizard at first because of his ability to control water in strange ways. As they try to capture him, Percy has to prove his good intentions. Meanwhile, some of the demigod enemies see this as an opportunity to attack while Percy is distracted. And then, of course, there could be a prophecy that ties the demigods and the wizards together in this strange situation.
One idea could be Percy getting involved with the Order of the Phoenix somehow. Maybe he's sent on a mission by the gods to help them out. He could use his water powers in new and creative ways within the wizarding world, like using water to put out magical fires or even to disrupt dark magic spells that rely on dryness.
Well, a cool plot might be that the phoenix is actually a reincarnation of an ancient Greek hero. Percy has to figure out who it was in its past life and how it ties into the current threats they face. The phoenix could also have the ability to travel through different realms, and it takes Percy on a journey to places he's never been before, like hidden parts of the Underworld or forgotten islands full of mythical creatures.
Another concept could be a Phoenix being reborn as a human - like form and becoming Percy's ally. They team up to fight against a new threat that neither could face alone. The Phoenix - human hybrid could have special powers that combine the best of both worlds, and their friendship could be a central theme in the fanfic.
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 interesting plot idea could be exploring the backstories of the minor characters in the Order more deeply. For example, what made them join the Order and their personal struggles during the fight against Voldemort.
One idea could be Percy having to balance the powers of chaos and order within himself. Maybe he discovers a new ability that is a blend of both, like creating temporary portals that are stable due to order but lead to chaotic realms. Another plot could involve Percy being the key to preventing a war between the forces of chaos and order. He has to convince both sides that co - existence is possible.
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.