One idea could be Percy discovers a new power related to the night, like the ability to manipulate shadows for protection or attack. Maybe he can use it to hide his friends during a battle against powerful monsters.
A really interesting plot could be that Percy is turned into a sort of 'night - version' of himself. His appearance changes, and he gains powers that are more attuned to the night. He has to learn to control these new powers while also dealing with the fact that his friends are a bit wary of this new him. He then uses his new form to fight an ancient evil that only he in this new state can defeat, perhaps an evil that thrives in the darkness and has been awakened by some dark magic.
One idea could be Percy exploring his new powers as the 'Son of Night'. Maybe he discovers abilities like controlling shadows to his advantage in battles against monsters. Another could be a story where he has to journey into the Underworld to retrieve something important for his new identity.
Well, perhaps in the fanfiction, the Angel of God is a messenger to Percy, warning him of a great catastrophe that will befall the demigod world. Percy then has to gather all the demigods and prepare them for this threat. The angel could also have the power to bestow temporary divine abilities on Percy and his friends, which they use to battle an army of dark creatures. Or it could be that the angel is actually a lost soul from a previous era, and Percy has to solve the mystery of its past to get its help in saving his world.
One idea could be that the Evil God manipulates Percy's friends against him. For example, making Annabeth doubt Percy's loyalty through false visions. Another could be that the Evil God corrupts the demigod world's magic, turning it against the heroes. Maybe the water Percy usually controls starts to harm him instead of help.
Maybe Percy is chosen by the Fates to be their champion in a battle against an ancient evil that threatens to disrupt the very fabric of fate. He has to team up with some unlikely allies, like minor gods who are also concerned about the balance of fate. He would have to learn new skills and face challenges that test his limits.
One idea could be Percy having to face a new and powerful monster that is a combination of multiple mythological creatures. This would test his skills and force him to come up with new battle strategies.
Well, a plot could revolve around Percy accidentally getting this 'God of Sex' title and trying to figure out how to get rid of it. He might go on a quest to find a way to reverse it. Along the way, he meets other characters who either want to use his power for their own gain or help him out of the mess. It could also involve some comical situations where his friends misunderstand his new powers and get into all sorts of awkward situations.
One idea could be Percy having to protect a group of special children who have unique powers related to the Greek gods. Maybe they are descendants of minor gods and are being hunted by some ancient monsters. Another plot could involve Percy teaching these children about their heritage and how to control their powers in a world that doesn't fully understand them.
Perhaps Percy as the God of Justice has to judge the actions of the Olympian gods themselves. This would create a lot of tension as he tries to hold them accountable for their past misdeeds. Or he could be in charge of a special court for demigods who have committed crimes but have valid reasons for their actions, and he has to balance justice and mercy.
Maybe Percy is chosen by the God of Nature (in this fanfiction) to go on a quest to find the missing seeds of the world - trees that once grew so large they held up the sky. Along the way, he meets new mythical creatures related to nature.
One idea could be Percy discovers a hidden dragon lair in Camp Half - Blood. Maybe the dragons were there all along, protected by ancient magic. Another could be Percy somehow gains the ability to communicate with dragons and has to use this new power to stop an evil force that's threatening both the demigod and dragon worlds.