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.
A cool plot could be that the balance of nature is thrown off because a powerful artifact that controls the seasons has been stolen. Percy, as the God of Nature in this fanfiction, has to track it down through different climates and face challenges from creatures who thrive in chaos.
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 Percy discovers he has a hidden power related to fate. Maybe he can see the possible fates of his friends and has to decide whether to interfere or let things play out. For example, he sees that Annabeth is in danger in a future quest and has to figure out how to change that fate without causing more chaos.
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.
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 plot idea could be Percy discovers his new power as the God of Destruction gradually. He might start by accidentally causing small disasters, like making a small pond dry up. As he learns to control it, he could face a great enemy that only his destruction powers can defeat. For example, a powerful titan who is immune to all other forms of attack.
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 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.
One idea could be Percy having to face a new, unknown threat that the Olympian gods are also powerless against. For example, a dark force from a long - forgotten realm. Another could be Percy going on a quest to find a lost artifact that has the power to reshape the relationship between the gods and demigods. Or perhaps Percy being forced to choose between loyalty to his friends and following the orders of the Olympian gods in a really tough situation.
A really interesting plot might be a story where Percy has to protect Artemis from a curse that is slowly draining her power. This could involve him going on a quest to find a cure, facing many challenges along the way, and developing a deeper understanding of Artemis. There could be some really cool moments where Percy uses his water - based powers in unique ways to help Artemis. Also, there could be some great character development as they interact more and more. This type of plot would also allow for exploration of the relationship between a male hero and a female goddess in the context of the Percy Jackson universe.