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Why don't you make a protagonist son of Zeus and legacy of Poseidon, besides making him very powerful it would still be consistent with the title, it's difficult to find Percy Jackson stories with a son of Zeus, Poseidon and Hades are outdated, make one crossover with Harry Potter and DxD, the story of Percy Jackson can take place at the same time as Harry Potter or take place later, like at the age of twelve the protagonist goes to Horwguats and he goes until he is fifteen when the story of Percy Jackson and the lightning thief, from then on you vary between both, at nineteen you finish percy jackson and start dxd together with the heroes of Olympus, it's just an idea.