Yes, there are likely fanfictions out there with such a concept. Fanfiction writers often like to create crossovers between different popular universes like Percy Jackson and Harry Potter. It's a fun way to combine the magic of Hogwarts with the demigod world of Percy Jackson.
One interesting storyline could be Percy and Harry teaming up to fight a common magical enemy that threatens both the wizarding world and the demigod world. Maybe it's a dark wizard who has somehow harnessed the power of the Greek monsters. Another could be Percy visiting Hogwarts and getting into all sorts of trouble because he doesn't understand the wizarding rules. For example, he might accidentally use his powers in front of Muggles - like if he tries to control water to put out a fire he caused, not realizing there are spells for that in the wizarding world.
The demigod world and the wizarding world would interact in some really wild ways. For example, the magic in Hogwarts might have an effect on Percy's powers. Maybe the protective spells around Hogwarts could make his water - controlling abilities act up. Also, the wizards would be fascinated by the demigod weapons like Percy's sword, Riptide. It's a weapon that can turn into a pen, which is something completely foreign to the wizarding world.
Perhaps Percy gets a strange magical summons to Hogwarts, and there he meets Harry. They figure out they're cousins. They then start to explore their family history together. Along the way, they realize that an ancient evil has been targeting their family for generations and they must use both magic and demigod powers to stop it. They also have to deal with the cultural differences between the two worlds, like Percy being used to the more physical and outdoorsy battles while Harry is more accustomed to spell - casting duels.
Yes, there are likely some fanfictions out there with this concept. The Percy Jackson and Harry Potter universes are both very popular, and fans often like to cross - over characters in interesting ways, such as making Percy and Harry cousins. You can try searching on fanfiction websites like Archive of Our Own or FanFiction.net to find such stories.
A really interesting plot could be that there is a prophecy in both worlds that only when Harry and Percy, as cousins, combine their powers can they defeat an ultimate evil. This evil could be something that has been lurking in the shadows of both universes, waiting for the right moment to strike. As they try to fulfill the prophecy, they have to learn about each other's powers and cultures. Harry would learn about the Greek gods and Percy would learn about magic spells. Along the way, they might face internal conflicts within their own communities who are skeptical or even against this union of the two different worlds.
You can start by giving them unique personalities. For example, one could be really shy but have a hidden strength. Then, put them in different situations to show how they grow. Like when they face a monster, the shy one might find the courage to fight.
Another character that could be included is Clarisse. Her tough - as - nails attitude could contrast with the niece's, and they could start off as rivals but eventually become friends. Also, Tyson, Percy's cyclops half - brother. He could be like a big, protective figure for the niece, and his unique abilities could come in handy during adventures or when the niece is in danger.
One idea could be that Percy's niece discovers she has a unique power that no one else in the family has. Maybe she can communicate with sea creatures in a different way than Percy. Another could be that she gets involved in a quest that was thought to be only for the main heroes in the Percy Jackson world, and she has to prove herself to the other demigods.
A rogue magician who has a vendetta against Percy's family. He sees Percy's niece as an easy target to get back at Percy. He uses his magic to manipulate the world around her, causing confusion and danger. He might also be trying to steal her powers for himself to gain more influence in the world of magic and mythology.