There might also be some of the gods as main characters. For example, Poseidon, Percy's father. He could be involved in protecting Percy and guiding him in the new setting of Olympus, while also dealing with his own affairs among the other gods.
He could be involved in some really cool quests. Like finding some ancient artifact that's hidden deep within Olympus. He'd probably make new friends among the minor gods and goddesses. And of course, there would be the constant threat of some ancient evil trying to take over Olympus, and he'd be right in the middle of the fight to defend it.
Grover could also be a major character. As Percy's best friend, he would probably be involved in trying to stop Percy if he thinks Percy has gone too far. Or, if he sees the corruption in Olympus, he may side with Percy. Grover's connection to nature means he could play an important role in the events that unfold, like using nature spirits to either protect Olympus or aid Percy depending on his stance.
Obviously, Percy Jackson is the central character. But often, Annabeth Chase also plays an important role. She might come to visit Percy on Olympus and get involved in his adventures there. Also, the gods themselves, like Poseidon (Percy's father), Zeus, and Athena, are frequently main characters as they interact with Percy in the Olympian setting.
Maybe there's a plot where the demigods are trying to improve the living conditions on Olympus. Since it's a place full of powerful beings, it might be in a bit of chaos. So they could be working on things like building better homes or creating a more organized system for the lesser - known gods.
The main characters include Percy Jackson, who is a well - known demigod. Then there's Jason Grace, a Roman demigod with his own set of powers and challenges. Piper McLean, who has special abilities and plays an important role in the group's adventures. Annabeth Chase, Percy's girlfriend, is also a significant character. She is smart and brave, often helping the group outwit their enemies with her strategic thinking.
Obviously, Percy would be one of the main characters. And probably the Olympian gods like Zeus, Poseidon, Athena etc. would also play important roles as he is raised in their domain.
Percy himself is always a popular choice. You can explore his growth, his relationships, and his new adventures. Another is Annabeth. You could write about her architectural projects in relation to the Olympian world or her further adventures in wisdom - seeking.
Percy himself is always interesting. In fanfiction related to The Staff of Olympus, he might have new challenges and growth. Another is Annabeth. Her intelligence could be used in unique ways to figure out the mysteries of the staff. Grover can also be really cool as he might use his nature - related powers in unexpected ways in relation to the staff.
It could be a new demigod who has a grudge against the Olympians. This demigod might be using Percy as a tool to carry out their assassination plans.
A primordial being, like Chaos or Nyx, could also be the main antagonist. If they feel that the Olympians have become too powerful or are misusing their power, they might decide to wipe out the entire demigod race. Percy would then have to find a way to either convince them to spare the demigods or find a way to defeat them, which would be extremely difficult as they are among the most powerful entities in Greek mythology.