Maybe Percy will face new challenges in college like dealing with difficult classes. He could also meet new friends or enemies. And perhaps he'll discover new powers or secrets related to his special nature.
The plot could center around Percy's struggle to fit in at college while still being a hero. He may find that the other students don't understand his need to disappear suddenly when there's a monster attack. He could also get involved in some sort of college - wide mystery that has roots in the ancient world. Like, strange symbols start appearing all over campus that only he can decipher because of his knowledge of Greek and Roman mythology. And he has to team up with some unlikely allies, like a nerdy professor who turns out to have a hidden knowledge of the demigod world, to solve the mystery and save the college from an impending doom.
There could be a really smart professor who suspects there's something different about Percy. Maybe a roommate who is into all kinds of strange and supernatural things without actually knowing the truth about Percy.
There could be a new demigod who transfers to Percy's college. This demigod could have powers related to nature, like being able to make plants grow instantly. They would become Percy's ally in dealing with the campus - based mythological threats. Another new character could be a strict dean who is always on Percy's case, not realizing that Percy is actually protecting the college from real danger. And there might also be a mysterious janitor who actually has a deep connection to the underworld and gives Percy cryptic warnings from time to time.
Percy going to Ilvermorny would be a wild ride. First, he'd be amazed by the castle itself. The four houses would be a new concept for him. He might be sorted into a house based on his heroic nature, like Thunderbird perhaps. He'd introduce the idea of Greek gods to the students there, which would cause quite a stir. And his relationship with Annabeth would be put to the test as they are separated by different magical institutions.
He might struggle at first with the academic part. Naruto is used to physical challenges in his ninja world. College courses like math or literature could be really tough for him. But his determination would probably make him study hard and gradually improve.
He might meet some talented witches and wizards. Maybe there would be a student who is an expert in magical beasts and they could share stories about their encounters with strange creatures.
There's a chance that Percy meets the personification of Tartarus itself. It could be a shadowy, all - powerful being that tries to break Percy's spirit. Percy, being the brave hero he is, would have to resist and find a way to either outwit or overpower this entity in order to continue his journey in Tartarus.
Perhaps Percy will meet a group of environmentalists. Given San Francisco's reputation for being environmentally conscious, they could be fighting against some sort of sea - related pollution that Percy gets involved in. He might use his powers and knowledge to assist them, and in the process, make some good friends.