I think many of the demigods who have felt oppressed or neglected by the current Olympian system would support Percy. Those from minor cabins who have never really had a voice in Olympus. Additionally, some of the more benevolent gods who see the potential for positive change in Percy's leadership might also offer their support, like Poseidon, his father, for obvious reasons, and perhaps Athena, if she sees the long - term benefits for Olympus.
His friends from Camp Half - Blood would likely support him. Annabeth, with her strategic mind, would be a great asset. Grover, too, would stand by Percy. Also, the younger demigods who have seen Percy's heroic deeds would probably back him as they look up to him.
In fanfiction, if Percy Jackson takes over Olympus, he might bring a new order. He could reform the archaic rules of the gods. For example, he might ensure fairer treatment of demigods, like better training and protection. Also, he could change the power dynamics among the gods, making them more accountable for their actions.
If Olympus read Percy Jackson fanfiction, the gods might be either amused or offended. Some fanfictions might portray them in a way that they don't like, and they could potentially take revenge on the author (if they could reach our world). But on the other hand, they might find some of the creative interpretations quite funny.
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.
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.
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.
His friends like Ron and Hermione would surely support him. They have been through a lot together, and they believe in Harry's leadership. Also, the Weasley family in general would be behind him. They have always been on Harry's side in his battles against the dark forces.
Annabeth Chase is likely to be one of the supporters. She is smart and strategic, and she can come up with plans to help Percy. Grover Underwood, Percy's satyr friend, might also be there. He can use his nature - related powers to help in the fight. Maybe some of the other demigods from Camp Half - Blood like Jason Grace or Piper McLean could also show up to support Percy.
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.