One idea could be a story where the Little Big Three go on a quest together in a new, uncharted part of the mythological world. Maybe they encounter a forgotten god who has the power to change the balance of power among the Olympians.
A great plot idea could be Percy having to deal with the jealousy of other demigods because of his special status as the 'brother' of the Big Three. This could lead to some tense relationships and even battles within Camp Half - Blood. Or perhaps Percy is forced to make a difficult choice between loyalty to his mortal friends and family and the demands that come with his connection to the Big Three. Maybe there's a story where Percy uncovers a secret power that only he, as related to the Big Three, can access, but it comes with a great cost.
Well, a really cool plot for 'Percy Jackson Son of the Big Three Fanfiction' might be that Percy is sent on a quest where the rules of the demigod world are all messed up. For example, the Oracle gives him a riddle that seems impossible to solve at first. He also could find himself in a situation where he has to choose between saving his friends or fulfilling a prophecy that could save the entire demigod community. He might also have to deal with some internal conflict, like whether or not he should embrace more of his godly side which could potentially make him more powerful but also more distant from his human friends.
One interesting concept is the exploration of the relationships between the children of the Big Three. For example, how Percy, a son of Poseidon, interacts with the children of Zeus and Hades. Another concept could be the unique powers that the characters might develop in fanfiction that are different from the original series. Also, the idea of crossovers within the Percy Jackson Big Three fanfiction, like with other mythologies, can be really engaging.
One idea could be a fanfiction where Percy and his friends go on a new quest in a modern, urban setting. Instead of the usual mythological locations, they have to deal with monsters hiding in big cities. Another idea is a story that focuses on the relationships between the characters in a more in - depth way, like exploring the friendship between Percy and Grover during their off - quest times. And a third idea could be a time - travel fanfiction, where Percy gets sent back to the time of the ancient Greek heroes and has to interact with them while still holding onto his modern values.
One idea could be to explore Percy's life after he has fully mastered his powers. Maybe he goes on a quest to find a long - lost relative who also has demigod powers. Another could be a story where Percy and his friends have to deal with a new type of monster that is immune to their usual weapons and they have to find a creative way to defeat it.
One idea could be that his little sister discovers a new, hidden power within herself while on a quest with Percy. Maybe she can communicate with mythical creatures in a unique way that helps them solve a mystery.
One idea could be that Percy's little sister discovers she has a unique power that even Percy doesn't have. For example, she can communicate with the spirits of the old gods that have long been forgotten. This could lead to her going on a secret adventure to find out more about these ancient deities and how they can help in the fight against the new threats.
One plot idea could be Percy going on a quest to find a lost artifact that was once in the possession of his father Poseidon. This could lead him to encounter new mythical creatures and face challenges that test his powers in new ways. Another idea is Percy having to deal with the politics among the Big Three's children, where he has to make alliances and navigate through rivalries. Also, a plot where Percy discovers a hidden power within himself that is related to his heritage as the son of a Big Three god and has to learn to control it while also using it to save the world from a new threat.
One idea could be a crossover with the Harry Potter universe. Percy could visit Hogwarts and have to deal with both magical and mythological threats. Maybe he'd team up with Harry to defeat a common enemy that combines dark magic and ancient Greek monsters.
Well, you could write a fanfiction where Percy loses his memory and has to rediscover his powers and his place among the demigods. Or perhaps a story about how the minor gods in the Percy Jackson universe form their own alliance because they feel overlooked by the Olympians. You could also create a fanfic that focuses on Annabeth's journey if she was separated from Percy and had to fend for herself in a dangerous situation.