One idea could be Percy having to face a new and powerful monster that is immune to water. This would really challenge his typical combat strategies. Another could be Percy getting lost in a forgotten part of the Underworld and having to find his way out while dealing with various underworldly spirits.
One idea could be Percy going on a quest to find a lost artifact that has the power to restore a dying part of the godly realm. This would involve him facing new monsters and making unexpected alliances.
One idea could be Percy having to face a new, unknown monster that is immune to water. He'd have to rely on his other skills and the help of his friends in a completely different way.
Well, you could write a fanfiction where a minor character from the original 'Percy Jackson and the Blood of Olympus' gets their own story arc. Maybe a character who was always in the background but had a lot of potential. For instance, a demigod who discovers a new power related to the blood of Olympus that was previously unknown. They could then go on a journey to master this power and in the process, uncover secrets about the Olympians that were long forgotten.
One idea could be exploring a new prophecy that involves Percy and his friends having to team up with some of the lesser - known gods. This would introduce new characters and powers into the story.
One idea could be Percy discovers a new, hidden power within himself that is linked to an ancient prophecy. Maybe he can control the elements in a way no demigod has before.
A plot where the demigods have to team up with their mortal enemies, like some of the monsters, in order to face a greater threat that endangers both their worlds. This could lead to some really interesting character dynamics and moral dilemmas.
One idea could be exploring the relationships between the demigods in more depth. For example, how Percy and Annabeth's relationship evolves after the events of 'Blood of Olympus'. Another could be creating a new adventure for the demigods that involves a forgotten prophecy. Maybe they have to travel to a new, unknown part of the mythological world to fulfill it.
A really interesting plot could be Percy losing his memories due to a curse. His friends would have to help him regain his identity while also protecting him from enemies who see this as an opportunity to take him out. As he slowly remembers bits and pieces, he uncovers a conspiracy within the Olympian ranks.
A cool plot could be Percy getting lost in a different dimension that is related to the Greek mythology but has been forgotten. In this dimension, he has to find his way back while also helping the lost souls there. He could learn new powers and make new allies that would be useful when he returns to his own world.
In a Percy Jackson Warrior of Olympus fanfiction, a great plot could involve Percy being sent into the future of the demigod world. He sees the consequences of the current actions of the gods and demigods. For example, he discovers that a war between the different pantheons has led to the destruction of many demigod camps. So, he has to go back in time and try to change the events that lead to this future. This would involve him having to convince not only his friends but also the gods to change their ways, which would be no easy feat as the gods are often stubborn. He might also meet some of his descendants in the future, and they could help him understand what exactly went wrong.