A primordial being from the depths of Egyptian mythology that was once sealed away by Sobek. Now that Percy is in the picture, it senses an opportunity to break free. It could be a creature with the ability to manipulate the minds of mortals and demigods alike, making it a very dangerous foe for Percy to face.
Another option could be a corrupted demigod. Maybe a child of a minor Egyptian god who is jealous of Percy's status as Son of Sobek. This demigod could form an alliance with some dark forces and try to undermine Percy at every turn, creating all sorts of challenges for him.
Annabeth could be a great ally. She's smart and resourceful, and she could help Percy figure out the mysteries related to Sobek and his Egyptian connections. Her knowledge of architecture and strategy would be useful in any quests Percy undertakes.
A plot could involve Percy being sent on a mission to an ancient Egyptian underworld realm by Sobek. There, he meets various Egyptian deities and creatures. He has to use his skills as a demigod and his new powers from Sobek to navigate through the dangerous realm and return to the mortal world with important information for the safety of Camp Half - Blood.
Maybe Percy discovers a hidden power within himself that is related to Sobek. For example, he could gain the ability to control water in a more primal and fierce way, like Sobek does. He then has to learn to control this new power while facing off against an enemy who wants to use it for evil. This could lead to some intense battles and character growth for Percy.
Perhaps an ancient and evil creature that has a grudge against Oin. Since Percy is Oin's son, this creature targets him. It could be a monster that was once defeated by Oin and now wants revenge by going after his son.
It could be the Titans who see Percy as a threat. Since he is the son of Kronos, they might want to either use him for their own purposes or eliminate him to prevent any interference in their plans.
It could be a powerful Titan who escaped from Tartarus and wants revenge on Zeus by targeting his son Percy. This Titan might have unique powers that make him a formidable foe.
Perhaps an ancient sea god who was once an enemy of Pontus. This god could see Percy as a threat because of his heritage. He would use his own minions, like sea serpents and murlocs, to attack Percy and those he cares about. He might also try to corrupt Percy, tempting him with power to turn against his friends.
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.
A powerful Titan who sees Percy as a threat to his plans. Since Percy has the power of Zeus and Poseidon, he could potentially stop any chaos the Titan wants to create. The Titan might try to turn Percy's friends against him or find a way to strip him of his powers.
It could be the gods. They often have their own agendas and might see the son of chaos as a threat to their power. So they might manipulate Percy into betraying the son of chaos.