One possible plot twist could be Percy finding out that a fellow soldier is actually a spy from the enemy side. This would create a lot of internal conflict as he has to decide what to do about it.
Perhaps Percy discovers a hidden weapon or a secret military strategy that could change the course of the war. But he has to face many obstacles to get this information to the right people. In a fanfiction, this could lead to some really exciting action scenes where Percy has to fight his way through enemy lines to deliver the crucial information.
Well, one plot twist could be that Percy discovers he has a hidden connection to one of the major Trojan heroes. Maybe he finds out he's related to Hector in some way, which makes his fight against the Trojans much more complicated. He might struggle with his loyalty to the Greeks.
A really interesting plot twist might be that Voldemort and Percy are forced to team up against a common enemy. They realize that there is a greater threat that neither of them can face alone. This would be a huge change in their relationship as they are initially enemies. Or perhaps Percy discovers that he has some latent wizarding abilities due to his godly heritage, which completely changes the dynamics of the fight as he can now use both his demigod powers and magic.
Maybe there is a scientist or an inventor who becomes Percy's ally. This person could be creating new weapons or gadgets that Percy uses in battle. Their alliance could be crucial in giving Percy an edge in the fight. For example, the inventor might create a special type of grenade or a device that can jam the enemy's communication lines.
One possible plot twist could be that Sakura discovers a hidden weakness of Madara's during the fight. Maybe it's a chakra point that, when struck, temporarily paralyzes him, giving her an upper hand.
Well, it could be that Percy is fighting Ares to protect his friends. Maybe Ares has threatened Percy's loved ones in Camp Half - Blood, so Percy has no choice but to face the god of war.
Some of the Greek heroes like Achilles could be Percy's allies. Percy might learn a lot from Achilles about combat and heroism. And there could be some new characters created specifically for this fanfiction, like a young seer who can predict the outcome of battles and helps Percy make the right decisions.
The plot could involve Percy being sent on a mission to stop the Axis powers from using ancient artifacts with mythical powers. He discovers that the Nazis are searching for a powerful relic that could turn the tide of the war, and he has to race against time to find it first.
A possible plot twist could be related to her powers. If Percy had certain powers before, they could be altered or enhanced in unexpected ways after reincarnation into a girl. For instance, her physical strength might be combined with a new kind of magical ability that is unique to female characters in that fictional universe. Also, there could be a twist where she discovers a hidden connection to a female - only group or prophecy that she was unaware of as Percy. This could lead her on a whole new adventure and change the direction of the story.
One possible plot twist could be Percy's struggle to adapt to the new gender. For example, she might face challenges in how society now treats her. Maybe she was used to certain privileges as a boy that are now different. Another twist could be in her relationships. Her friends might react in various ways, some being very accepting while others are confused or even distant at first. And she could also discover new aspects of herself, like different interests or ways of expressing emotions that were suppressed before.
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.