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.
Perhaps Percy, as the God of the Earth, has to plant the seeds of a new magical forest to restore balance to the world. But an evil force is trying to stop him by sending monsters to attack him and the forest he is trying to create. He has to use his earth powers to defend and also enlist the help of nature spirits like dryads. He also discovers that this new forest has a connection to an ancient prophecy that could change the fate of the world.
A great plot idea could be Percy Jackson being transported to the world of God of War and having to adapt to its harsher rules. He'd have to learn to fight without relying solely on his water powers, as the enemies there are more fierce and less predictable. Maybe he gets mistaken for an enemy by Kratos at first and has to prove himself.
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.
One plot idea could be Percy Jackson being sent on a quest to the Norse realms by the Greek gods. There, he has to team up with a Norse god, like Thor perhaps, to stop a common enemy that threatens both the Greek and Norse worlds. Another could be Percy getting caught in a power struggle between different Norse gods and having to use his wits and demigod powers to survive. Maybe he discovers he has some unexpected connection to the Norse gods, like a long - lost relative who was part of both worlds.
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.