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.
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 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.
Well, you could have a fanfic where Percy is given a science homework about marine biology, and he uses his knowledge from being the son of Poseidon to write an amazing paper. He might even bring in some sea creatures from the ocean to show as examples, which of course causes chaos in the classroom. Another idea is that the whole demigod gang has to do a group project on mythology, and they each contribute their own unique experiences with the gods and monsters.
In a 'Percy Jackson tattoo fanfic', you could explore the idea of a tattoo artist who is actually a minor god or goddess. The story could follow Percy or another character as they go to get a tattoo from this divine artist. The tattoo could have powers that are both a blessing and a curse, like it gives the wearer enhanced strength but also attracts more monsters. And the process of getting the tattoo could be filled with mythological elements, like the ink being made from the essence of a magical creature.
One idea could be a story where Percy and Phoebe team up to face a new threat that neither of them could handle alone. Maybe it's a powerful monster that has the ability to control water and fire, so Percy's water powers and Phoebe's skills need to be combined.
One idea could be a fanfic where Percy has a dream that he's in an alternate version of Camp Half - Blood. In this dream, the cabins are arranged differently, and there are new campers with strange powers. Another idea is a dream where Percy dreams of his future, seeing himself as a teacher at Camp Half - Blood, passing on his knowledge and skills to the new generation of demigods. Or perhaps a fanfic where Percy has a recurring dream about an ancient prophecy that he has to figure out in his waking life.
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.
One idea could be that your inserted character is a new camper who has a special ability related to an ancient and forgotten Greek magic. This character then helps Percy and his friends in a battle against a revived mythological villain that is more powerful than any they've faced before.
You could write a fanfic about Percy and Annabeth in the future, like when they are adults. They could be building a new Camp Half - Blood type of place for demigods all over the world. But there are those who oppose their ideas, and they have to deal with politics and power struggles, all while maintaining their relationship strong through it all.
A good crossover idea is that there's a spell gone wrong in the wizarding world that starts to affect the boundaries between the mortal world and the world of the Greek gods. Hermione, being the brilliant witch she is, tries to figure out a solution, and Percy, with his knowledge of the Greek world, gets involved. They could travel through different dimensions together, facing challenges from both magical creatures and mythical beasts.