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.
Perhaps there is a magical object that has been lost for centuries and it holds the key to the true power of their family. Harry and Percy, upon discovering their relation, set out on a quest to find this object. They have to travel through both the wizarding world and the demigod world, facing various challenges specific to each. In the wizarding world, they might have to deal with dark wizards trying to steal the object for their own gain. In the demigod world, they could face mythical creatures guarding the object. As they progress, they also grow closer as cousins and learn more about the shared heritage that binds them.
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 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.
A plot could involve a competition between the two groups. Maybe the wizards challenge the demigods to a series of magical and physical tasks. This would lead to some really interesting interactions as they learn about each other's abilities. The demigods would be amazed by the wizard's spells and the wizards would be in awe of the demigods' powers.
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.
A plot where the villains from both universes team up. Voldemort and the Titans or Giants could join forces. This would force Percy and Harry to put aside their differences and work together. They would need to combine their unique powers and knowledge. Percy's ability to control water and his combat skills along with Harry's magic skills and knowledge of the wizarding world's secrets would be crucial in fighting this powerful alliance.
Well, they could be the result of some sort of cross - over experiment gone wrong (but in a good fanfic way). Maybe they have powers from both worlds but struggle to control them at first. For example, they might accidentally cast spells while trying to use their demigod powers. And they could be the bridge between the two groups of friends. The demigods are skeptical of wizards and vice versa, but the twins help them understand each other.
One idea could be that Harry and Percy meet during a magical convergence. They realize they can combine their powers to fight a new, powerful dark force that threatens both the wizarding world and the world of the Greek gods. For example, Percy could use his water powers to put out a magical fire created by a dark wizard, while Harry uses his magic to protect Percy from a monster attack.
One idea could be that the twins have powers from both the wizarding world and the demigod world. They can use magic spells like in Harry Potter and also have the abilities of a demigod from Percy Jackson. For example, they can control water like Percy and at the same time cast defensive spells. Another plot could be that they are sent on a mission to stop a threat that endangers both the wizarding world and the Camp Half - Blood. They have to use their combined knowledge and skills to succeed.
One idea could be that the demigods and wizards have to team up to fight a common enemy that is threatening both the magical and the mythological worlds. For example, a dark force that can manipulate both magic and the powers of the gods. Another plot could involve Percy and his friends getting lost in the wizarding world and having to navigate Hogwarts while also dealing with their own demigod issues. Maybe they discover that some of the magic in the wizarding world has its roots in the ancient Greek magic that the demigods are familiar with.
One plot idea could be that Percy and Harry discover they are long - lost brothers and have to combine their magical powers to defeat a new, powerful dark force that threatens both the wizarding world and the world of Greek gods. For example, they might have to find a way to stop a dark wizard who has made a pact with a rogue Greek god to gain ultimate power.