One interesting plot twist could be that the villains in both worlds are actually being controlled by a single entity that exists in a dimension between the Greek mythological world and Naruto's world. Percy, as the son of Hera, and Naruto initially think they are fighting separate battles. But as they start to notice similarities in the attacks and motives of the villains, they team up to find this hidden entity. When they finally do, they discover that it is a being created from the combined negative emotions of both worlds, and they have to use their unique powers together to defeat it.
Imagine this - Percy, thinking he's on a normal quest as the son of Hera, stumbles upon Naruto's world. But it turns out that Naruto's world is a sort of prison for the Greek gods, and they have been trapped there for centuries. Naruto and his friends have unknowingly been guarding the gods all this time. Percy now has to figure out how to free the gods without causing chaos in Naruto's world, and he has to convince Naruto to help him, which is difficult since Naruto doesn't trust these new 'powerful beings' from another world.
One possible plot twist could be that Percy, as the son of Hades and Hera, discovers a hidden power within him that can control both the underworld and the aspects of Hera's domain like marriage and family. This power could put him in the middle of a great conflict between the gods who are jealous of his unique abilities.
In some fanfictions, a really cool plot twist could be that Percy's Hela - given powers are slowly corrupting him. He starts to see the world in a different, darker way. His friends notice the change in him but don't know the cause at first. As he struggles with this corruption, he has to find a way to either embrace it and use it for good in a new way, or find a way to get rid of it. Also, imagine if Percy discovers that he has a half - brother or sister from Hela's side in the Norse world, and this new relative has a completely different view on life and power, which causes more drama and conflict for Percy.
Well, Percy could be sent on a quest by Hera to reunite broken families in the mortal world. This would be in line with Hera's role as the goddess of family. As he does this, he could encounter various monsters and challenges. Along the way, he might discover that some of these broken families are the result of the interference of other gods or mythical beings. Percy would then have to use his wits and powers (as the son of Hera) to not only fix the families but also deal with the root causes of their problems. This could involve battles, negotiations, and a lot of self - discovery for Percy.
One possible plot could be Percy discovering his new parentage of being the son of Zeus and Hera. He might struggle with the new identity as it goes against what he previously knew. For example, he could face challenges from the other demigods who are suspicious or jealous of his new status. He then has to prove himself worthy of this new heritage while also dealing with the complex politics among the gods.
One interesting plot twist could be that Percy discovers he has some of Achilles' more negative traits along with his powers. For example, Achilles was known for his pride, and Percy might start to struggle with excessive pride that gets in the way of his relationships with his friends at Camp Half - Blood. Another could be that the gods, fearing Percy's new connection to Achilles, send out a series of trials that are specifically designed to test his loyalty and his ability to control the power of Achilles. Maybe he has to face off against a powerful monster that can only be defeated if he uses his Achilles - like invulnerability in a very specific and dangerous way.
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.
One idea could be Percy discovering his connection to Hera later in his adventures and having to deal with the reactions of his friends. Another could be him having unique powers inherited from Hera that set him apart from other demigods.
In the stories, the revelation about Luke being a traitor is a big plot twist. He was initially seen as a friend and mentor to Percy. But later, it turns out he has his own dark motives and is working against the gods, which adds a lot of complexity and danger to the story.
A plot idea might be that Percy is magically transported to a time when Poseidon and Hera had a major disagreement in the past. He has to use his wits and powers as Poseidon's son to prevent this rift from growing and causing chaos in the present. Also, he could be given a task by Hera to rescue one of her favored heroes who has been trapped by a sea monster under Poseidon's domain. This would put Percy in a tricky situation where he has to balance his loyalty to his father and the task given by Hera. He might also encounter a group of demigods who are descendants of both Poseidon and Hera in some way, and together they have to face a common enemy.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.