One interesting plot twist could be that Percy's control over the ocean is suddenly taken away by a dark magic spell. Then he has to find a way to get it back, perhaps by going on a quest to find a long - lost artifact or by making a deal with a mysterious sea - dwelling being. Another twist might be that the ocean starts to rebel against Percy's control. The sea creatures turn on him, and he has to figure out why and how to fix it.
In some fanfictions, his power develops as he forms new alliances. If he teams up with a water nymph or a merman who has different knowledge about the ocean, he can learn new ways to use his power. His emotional state can also play a role. If he becomes more in tune with his love for the ocean and its inhabitants, his power could grow stronger, allowing him to do things like create more complex oceanic structures or calm larger areas of stormy seas.
One interesting plot twist could be that the Minotaur is actually a misunderstood creature under a curse. In some fanfictions, it might be revealed that he was once a heroic figure before being transformed. Another twist could be Percy having a secret power that he discovers while fighting the Minotaur that changes the whole dynamic of their encounter. For example, he might be able to communicate with the Minotaur's mind and find out its true motives.
A really cool plot twist might be that the reincarnated character realizes they are not just reincarnated into the Percy Jackson world randomly. They are part of a prophecy that was long forgotten. Their actions in this new life will either save or destroy the world of demigods. This would add a layer of high stakes and mystery to the fanfiction.
Another plot could involve Percy having to deal with a powerful sea monster that is not from Greek mythology but from some other ancient culture's sea lore. He has to use his skills and the help of his friends in a new and different way to defeat it.
One interesting plot twist could be Percy being secretly trained by an ancient and forgotten god. This god might have a hidden agenda that Percy gradually uncovers throughout the fanfiction.
One interesting plot twist could be that Annabeth discovers a hidden power within herself that has nothing to do with Athena. Maybe she can suddenly communicate with mythical creatures in a way no one else can. Another could be Percy finding out that he has a long - lost sibling who is actually on the side of the enemy, but then later switches sides. And a third one might be that the Mark of Athena is actually a fake, created by an enemy to mislead the demigods and lead them into a trap.
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 the gods are actually players in this 'game' too. They manipulate events not just as deities but also as players with their own scores and goals. Another could be Percy finding out that his real power is not what he thought, like he has the ability to reset the 'game' world. And a third could be that some of the monsters are actually glitches in the game system, and Percy has to figure out how to deal with them in an unexpected way.
In some fanfictions, there could be a twist where the gods are not as all - powerful as they seem. Maybe they are being manipulated by a greater, unknown force. This would force Percy and his friends to re - evaluate their relationships with the gods and find new ways to deal with the threats to their world. It would also change Percy's destiny as he might have to take on a more independent role in saving the world without relying as much on the gods' help.
One possible plot twist could be that Percy discovers a hidden power within Tartarus that he can use to his advantage. Maybe it's a long - lost magic that only the darkest part of the Underworld can unlock.