An ancient spirit that was trapped for centuries and Percy accidentally releases it. The spirit has a grudge against the Olympians but also sees something special in Percy, so it's a very complex relationship. It could add a lot of mystery and danger to the fanfic.
A young demi - god who looks up to Percy and wants to be his apprentice. This character could bring some new energy and naivete to the story, and Percy would have to take on a mentoring role.
A powerful witch from a far - off land who has her own motives for getting involved with Percy and the Olympians. She could be an ally at times and a foe at others, depending on her own goals.
Well, Percy is surely one of the main characters. He is likely to be the central figure around which the story revolves. He has his unique powers and personality that drive the plot forward.
A possible new character could be an oracle - like figure who is not from the traditional Delphi but is a seer on Olympus. This character could give Percy cryptic prophecies that are related to the future of Olympus and the demigod world. There might also be a group of centaurs who are specifically trained to protect Percy and serve as his advisors, different from the ones at Camp Half - Blood.
A new demigod from a really rare godly parentage, like a child of Hypnos, the god of sleep. This character could have unique powers related to dreams and illusions and could be an ally or an adversary to Percy. Another could be a centaur who has been exiled from his herd for some reason and has a vendetta against the Olympians. He could be a source of knowledge for Percy about some of the more hidden secrets of the gods.
Obviously Percy would be one of the main characters. There might also be other characters from the Percy Jackson series like Annabeth. Maybe the gods of Olympus would also be important characters as they are the ones whose home, Olympus, is being destroyed.
Perhaps the plot involves Percy using his new princely powers to bring about positive changes in Olympus. He could work on improving relations between the gods and demigods, for example. Since he has always been a bit of an outsider among the demigods because of his unique abilities, this new role could give him the opportunity to bridge the gap between the two groups. He may also have to face some sort of conspiracy within Olympus that threatens his position as prince.
Maybe there's a plotline where Percy discovers a long - lost relative who has some crucial information about his past or his powers. This could lead to a whole new adventure as he tries to piece together the mystery. For example, this relative could be from a forgotten branch of his family tree that has a special connection to an ancient power source in the Olympian world.
One idea could be Percy having to deal with a new, powerful enemy that even the Olympians are afraid of. He has to use his unique skills and form new alliances.
Grover might also be an ally. His connection to nature could be useful in Percy's new role. For example, he could help Percy understand any nature - related aspects of Olympus or deal with nature spirits in the realm. Also, some of the more benevolent gods like Poseidon (Percy's father) and Athena (Annabeth's mother) might side with Percy, at least to some extent, as they have their own interests in seeing Percy succeed in maintaining order in Olympus.
There could also be some ancient monsters or spirits that are tied to Olympus' existence. They might rise up to defend it because their own survival is linked to the continued existence of Olympus. These could be powerful beings that Percy has never faced before, adding an extra layer of danger to his quest to destroy Olympus.
A new and powerful Titan who has been hiding and gathering strength all this time. He could be jealous of the Olympians' power and wants to take over. Percy would have to figure out his weaknesses and stop him before he causes too much destruction.