A more complex plot could be that there is a prophecy that says only by the combined forces of Percy Jackson and the Class of the Titans can a great disaster be averted. However, there are mistrust and differences between the two groups at first. So they start off on the wrong foot, but as they face various challenges together, they gradually learn to trust and rely on each other to fulfill the prophecy.
One idea could be that Percy and the Class of the Titans have to team up to fight a new, powerful enemy that is a threat to both the Greek world and the Titans' realm. Maybe a primordial being that has been awakened and is causing chaos everywhere.
One idea could be Percy having to face a new type of Titan - one that can control time. He'd have to figure out how to break free from the time loops the Titan puts him in. Another could be Percy teaming up with some unlikely allies from the Titan side who have a change of heart and want to stop the more evil Titans. Maybe a Titan who was forced into the fight but doesn't really believe in the cause.
A really interesting plot could be that there is a tournament where demigods and shinobi compete. Naruto and Percy are the representatives of their respective groups. Along the way, they learn about each other's cultures and powers. They also discover that there are some dark forces trying to manipulate the tournament for their own evil purposes. So, they have to put aside their differences and work together to expose and defeat these forces.
An ancient and powerful Titaness who was banished long ago could return. She is angry at both the Greek gods and the current Titans. So she decides to use Percy as a pawn to start a war between the two groups so she can emerge victorious and rule over all.
Perhaps Percy could have a dream during Greek class about an upcoming prophecy related to the class. In the dream, he sees that a certain ancient Greek artifact hidden in the school is going to be discovered, and it will either bring great danger or great power. So, he spends the rest of the class trying to figure out how to stop or find it first. Also, he could get into an argument with a teacher who doesn't believe in the real existence of the Greek gods, but Percy has to prove it without revealing his true identity as a demigod.
Perhaps there could be a situation where the boundaries between the wizarding world and the world of Greek gods start to break down. Harry and Percy find themselves in a strange new world that is a mix of Hogwarts and Camp Half - Blood. They must figure out how to restore the boundaries while also dealing with the internal politics of both worlds. There could be some interesting character interactions, like Hermione trying to understand the Greek mythology and Annabeth being fascinated by the magic spells.
One idea could be a quest where Magnus and Percy have to team up. Maybe there's a new threat that combines the Norse and Greek mythologies, like a monster that has the powers of both a Greek hydra and a Norse dragon. They would have to use their different skills and knowledge to defeat it.
In a fanfic, perhaps Percy and his friends accidentally stumble upon Medusa's lair while on a quest. But instead of immediately attacking, Medusa offers them a deal. She says she will give them a valuable clue to complete their quest if they help her get revenge on one of the minor gods who wronged her in the past. Percy and his friends then have to decide whether to trust Medusa or not.
A really interesting plot could be that the reader has the ability to see the future in dreams, but the dreams are very cryptic. Percy and the reader have to work together to decipher these dreams to prevent a great disaster from befalling Camp Half - Blood. This would involve a lot of mystery - solving, and their relationship would develop as they rely on each other more and more to save the day.
Another plot could involve Percy's friends getting accidentally transported to a different realm in the Percy Jackson universe. They have to rely on their friendship and the skills they've learned from Percy to survive and find their way back. For example, they might end up in the Underworld and have to navigate past various challenges without Percy at first.
One idea could be a mortal accidentally stumbling into Camp Half - Blood and having to adapt quickly to the world of demigods. Maybe they have some hidden power that only reveals itself in this new environment.