Maybe it could be a rogue demigod who is jealous of Percy. This demigod could be working with some dark forces and tries to make Percy's situation worse while he's holding up the sky. They could spread rumors among the demigod community to turn others against Percy, making it harder for his friends to get help.
It's possible that the main antagonist is an ancient and malevolent spirit that has been trapped for ages and sees Percy's predicament as an opportunity. This spirit could be influencing the minds of some of the demigods and creating obstacles for Percy's friends. It might also be able to directly attack Percy while he's holding up the sky, trying to make him drop it.
A group of rogue sky spirits might serve as the main antagonists. They are jealous of Percy's status as the 'Son of the Sky' and want to steal his powers. They could be constantly causing chaos in the sky, like creating dangerous storms and attacking flying creatures. Percy would have to figure out their motives and stop them before they cause too much damage. These sky spirits could have their own hierarchy and leader, making them a formidable foe.
The main antagonist could be a powerful sorcerer who has found a way to enter the Percy Jackson game world. He has his own motives, perhaps he wants to steal the powers of the demigods for himself. He's able to manipulate the game's code to his advantage, creating illusions and false paths for Percy and his friends to follow. Or it could be an ancient spirit that was sealed away in the game but has been released. This spirit is angry at the gods and demigods and wants to destroy everything they hold dear. Also, a group of bandit demigods could be the main antagonists. They don't follow the rules of the game or the real - world demigod society and are causing chaos everywhere they go, trying to steal valuable items from both the game world and the real - world locations.
A powerful sorcerer from another realm who uses assassins as tools. This sorcerer could see Percy as a threat to their plans of taking over the demigod world. They send their best assassins after Percy and also try to manipulate other demigods against him. They are extremely cunning and have a vast array of dark magic at their disposal.
Nyx could be a great main antagonist. She's a primordial goddess of the night and Tartarus is her domain in a way. She might see Percy and his friends as intruders and try to stop them from leaving or achieving their goals. She could use her power over darkness and the unknown to create all sorts of obstacles for them.
The main antagonist could be a powerful Titan who wants to use Percy as a weapon to overthrow the Olympian gods. The Titan might see Percy's unique powers as a weapon as the perfect tool to gain ultimate power and reshape the world according to his own desires.
It could be a new, powerful Titan that has emerged from the depths of Tartarus. This Titan might be seeking revenge on the demigods for imprisoning the other Titans and sees Hylla and Percy as the biggest threats.
Well, a great plot for a 'Percy Jackson holding up the sky fanfic' might involve Percy communicating with the gods while holding up the sky. Maybe he makes a deal with one of the gods in exchange for some help. Or perhaps he has visions of the future of the demigod world while in this state, and he tries to convey these visions to his friends through some magical means.
It could be an ancient sea god who has been forgotten but now wants to reclaim his former glory. He sees Percy's ocean soul as a key to achieving that and will stop at nothing to get it from him, causing chaos in the process.
It could be a new demigod who is jealous of Percy's newfound super speed and wants to take it for himself. He becomes the main obstacle for Percy throughout the fanfic.
Perhaps it could be a new evil force created as a result of Percy's sudden god - born existence. His new - found power could have unbalanced some ancient magic or prophecy, and this new evil entity rises to oppose him and restore the old balance. It might be a being made of dark magic that has a vendetta against Percy for disrupting the natural order of things.