One idea could be Percy having to use his new powers of light to battle against an ancient darkness that is threatening to engulf the world. He might have to journey to the underworld to find the source of this darkness and use his light to purify it.
Perhaps Percy, as the God of Light, has to deal with other gods who are jealous of his new status. This could lead to political intrigue and power struggles among the gods. He would have to use his wits as well as his powers to maintain his position and keep the peace in the divine realm.
Perhaps in the fanfiction, the Elder Gods start to interfere with the modern world of demigods more directly. Percy and his friends notice strange phenomena, like the weather going haywire and magic behaving erratically. They soon find out it's because an Elder God is trying to regain its former power and is using the chaos to break through the barriers that keep it in a weakened state. Percy then has to find a way to stop it by using his unique connection to water and the help of his father, Poseidon.
One idea could be Percy having to face a new type of demon that is immune to water, his usual weapon. So he has to find a new way to fight. Another could be Percy being tricked into serving the God of Demons for a while and then realizing his mistake and trying to break free.
One idea could be exploring a new adventure where Percy has to team up with a minor god that has been forgotten. They could go on a quest to regain the forgotten god's power and status in the pantheon.
Perhaps Percy, as the God of Wisdom, has to deal with a situation where his wisdom is being questioned by the other gods. They don't believe that a former hero like him can handle the responsibilities of wisdom. So he has to prove himself by solving a complex problem that has been plaguing Olympus for centuries. He uses his unique perspective as a demigod - turned - god to come up with a solution that no one else could think of.
One idea could be Percy having to prove his divinity in a new way. Maybe he has to face a trial set by the gods where he can't use his usual powers but has to rely on his wits and faith. Another could be Percy interacting with a different pantheon's deities who mistake him for one of their own sons of god. He'd have to navigate their culture and rules.
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.