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.
There's a possibility that it's a powerful sorcerer or witch who has cast a spell on Percy. This villain wants to use Percy to assassinate the Olympians so they can take over the power in the Olympus realm. They might be hiding in the shadows, pulling the strings and making Percy's life a living hell.
Perhaps the story involves Percy being framed as an assassin of Olympus. He then has to clear his name while dealing with the real assassin who is causing chaos among the gods and demigods. The plot might also include Percy's journey to discover who set him up and why.
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.
It's not likely that the actual Olympus (in the context of the Percy Jackson series being a place of the gods) reads fanfiction. Fanfiction is a creation of the fans in our world, and there's no indication in the books that the gods or Olympus as an entity would engage with it.
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.
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.
His friends from Camp Half - Blood would be his allies for sure. Annabeth, with her strategic mind, would be invaluable. Grover, the satyr, could help Percy communicate with nature spirits which could be useful. But also, some of the minor gods who feel left out by the major Olympians might side with Percy. For example, Hecate could use her magic to assist Percy, seeing him as a chance for a new order in the Olympian world. There's also Tyson, Percy's cyclops half - brother, who would be a strong and loyal ally.
Annabeth could definitely be one of his main allies. Her intelligence and strategic planning would be invaluable in a fight against Olympus. Also, Grover might be there, using his skills as a satyr to gather information and support from other nature - related beings. Some of the less - well - known demigods from Camp Half - Blood could also join in, seeing Percy as a chance for a new order.
One great fanfiction is 'The New Quest of Percy Jackson'. It continues Percy's adventures in a very believable way, with new challenges and old friends by his side. The author really captures the essence of the original series.
A lesser - known but really interesting fanfiction is 'Annabeth's Quest for Wisdom'. This story follows Annabeth as she goes on a solo quest to gain more knowledge and wisdom. It shows a different side of her character, her determination and her thirst for knowledge in a very engaging way.
Sure. Given the popularity of the 'Percy Jackson' books, there are all kinds of fan - created stories out there. Some fans like to twist the original character traits and storylines. Making Percy a hired assassin is a way to put a new spin on the character. It could involve him using his demigod powers in a more morally ambiguous way compared to the original stories.