One interesting plot twist could be Percy finding out he has a hidden power related to Hades' realm that he didn't know about before. Maybe he can control a special type of underworld creature that no one has ever seen. Another could be Percy having to face off against a false heir to Hades' throne who is trying to frame him for crimes in the underworld.
A great plot twist might be that Percy discovers that his adoption by Hades was part of a much larger prophecy. This prophecy could involve saving the entire underworld from a great threat that only he, as an outsider adopted into the family, can face. It would add a whole new layer of mystery and importance to his character.
One idea could be Percy having to navigate the Underworld politics due to his relation to Hades. He might face challenges from other underworld creatures who don't trust him. Another could be Percy using his newfound powers as the son of Hades to fight against an ancient evil that threatens both the mortal and underworld realms. Or perhaps he has to deal with the stigma of being Hades' son in the demigod community and prove himself worthy.
Another plot could involve Percy's relationship with his father, Hades. Maybe Hades has some grand plan for Percy that he doesn't fully understand at first. Hades could be trying to use Percy to gain more power in the world of the gods, but Percy resists and has to fight against his father's wishes while also dealing with the internal conflict of not wanting to completely turn his back on his family.
A plot could revolve around the son of Hades having to face a new and powerful monster that is specifically targeting him because of his heritage. This would lead to a series of adventures where he has to use his unique powers from Hades to survive and protect his friends. For example, he might be able to control the undead in new and unexpected ways to fight this monster.
Perhaps a really interesting plot twist could be that Percy's adoption by Erebus reveals a long - lost prophecy. This prophecy states that Percy, with his combined heritage of being Poseidon's son and Erebus' adopted son, is the key to either the destruction or the salvation of the entire Greek mythological world. This would put Percy in a very difficult position as he tries to figure out how to fulfill the prophecy in the best way possible, all while dealing with the reactions of other gods, demigods, and mythical creatures.
One interesting plot twist could be that Percy, as the son of Vulcan, discovers a hidden power within him that's related to forging magical weapons. For example, he might be able to create a weapon that can only be wielded by those with a pure heart, and this weapon becomes crucial in a battle against an ancient evil. Another plot twist could be that his relationship with his father, Vulcan, is not as straightforward as he thought. Maybe Vulcan has been secretly guiding his enemies to test Percy's strength. Also, perhaps Percy finds out that he has a connection to an ancient prophecy that was long forgotten in the world of Vulcan's descendants, and this prophecy puts him in a position where he has to make a difficult choice between saving his friends or fulfilling a greater destiny.
In some fanfictions, a cool plot twist could be that Percy's connection to Kronos gives him the ability to travel through time. He could use this power to go back and change certain events that led to Kronos' rise or fall. But this also comes with risks, like accidentally changing things for the worse or getting trapped in a different time period.
One possible plot twist could be that Percy's demigod powers interact in unexpected ways with Loki's magic. Since Percy has control over water and Loki is known for his trickery, perhaps Loki uses Percy's powers to create illusions involving water, like making a dry desert seem like an ocean to deceive enemies.
One idea could be Percy discovering new powers related to being the son of Hades. For example, he might be able to control the shadows in a unique way that no other demigod can. He could use this power to spy on enemies or to travel quickly through the underworld.
Maybe the son of Hades in the fanfic has a special bond with a particular type of monster that is usually associated with death, like a Cerberus - like creature. Percy Jackson and his friends are initially scared of this bond but then realize it could be used to their advantage. For example, the son of Hades could use this bond to control the monster and have it help them in a battle against a common enemy. The fanfic could also explore how the son of Hades tries to fit into the world of Camp Half - Blood while being different from the other demigods.
One possible plot twist could be Percy's struggle with his new identity. Since Zeus is the king of the gods, Percy might face a lot of envy and even threats from other demigods or minor gods who see him as having an unfair advantage. Another twist could be Percy's relationship with his old friends at Camp Half - Blood. They might feel betrayed or confused as to why Percy has been adopted by Zeus instead of Poseidon.