One idea could be Percy discovers a new power related to the elements and has to learn to control it quickly to save his friends from a powerful enemy. For example, if he gains the ability to manipulate fire, he might accidentally set things on fire at first.
A great plot could be that an ancient evil is awakening and it can only be defeated by the combined powers of all the elements. Percy, as the 'God of Elements' in the fanfic, has to journey across different realms to gather the essence of each element. Along the way, he meets new allies and enemies, and has to overcome many obstacles related to the nature of each element.
Perhaps Percy is chosen by the gods to be the 'God of Elements' in a special ceremony. But this new title comes with many challenges. He has to deal with other demigods who are jealous of his new status. He also has to figure out how to balance the powers of all the elements within himself.
A cool plot could be that the God of Death has gone rogue and is starting to take souls before their time. Percy, with his friends, has to find a way to stop the god. They would have to search for ancient artifacts that can control or at least influence the God of Death's actions. They'd face many challenges, like traps in the Underworld and battles with deathly minions.
Maybe in the fanfic, Percy teams up with some unexpected allies, like a group of minor gods who also feel threatened by the more powerful gods' actions. Together, they form a plan to slay a corrupt major god, which involves traveling to different realms and facing various mythical creatures along the way. The story could explore themes of friendship and betrayal as they work towards their goal.
One idea could be that Percy has to hide his Titan heritage from the other demigods at Camp Half - Blood. He discovers new powers related to his Titan side that are different from his godly powers. For example, he might have the ability to control certain aspects of nature in a more primal way compared to his Poseidon - related powers.
In a fanfic, perhaps Percy and his friends accidentally stumble upon Medusa's lair while on a quest. But instead of immediately attacking, Medusa offers them a deal. She says she will give them a valuable clue to complete their quest if they help her get revenge on one of the minor gods who wronged her in the past. Percy and his friends then have to decide whether to trust Medusa or not.
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.