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.
A really cool plot could be that Percy is the God of Destruction but in a parallel universe. In this universe, the rules are different. He has to work with his alternate - self who is a very different kind of hero. Together, they combine their powers and knowledge to stop an inter - dimensional chaos. His destruction power is the key to closing the portals that are causing the chaos.
One idea could be Percy discovers a new power within him that is related to destruction but he has to learn to control it so as not to harm his friends. Maybe he accidentally causes some chaos in Camp Half - Blood at first.
One plot idea could be Percy having to face the Primordial of Destruction in a direct confrontation. Maybe the Primordial is awakened by an ancient evil spell and starts causing chaos in the demigod world. Percy has to gather allies from different camps and figure out a way to stop it before it destroys everything. Another idea is that the Primordial somehow infects Percy with a part of its destructive power, and he has to struggle not to let it take over him while also trying to find a cure. And a third plot could involve the Primordial creating illusions of destruction all over the world, and Percy has to travel through these illusions to find the real source and defeat it.
One possible plot could be Percy being bestowed with the powers of the God of Destruction accidentally. He then has to struggle with controlling these new, chaotic powers while still trying to protect his friends and the world. Maybe he accidentally causes destruction in places he loves, like Camp Half - Blood, and has to find a way to reverse it.
A possible plot would be that Naruto arrives during a time of unrest in the Percy Jackson world. His presence as the God of Destruction shakes up the balance of power. Some might see him as a savior, thinking his power can end the chaos, while others fear he will only make it worse. He would have to deal with the politics among the gods, like Zeus being suspicious of his power. Naruto could also encounter demigods who are both in awe and afraid of him. He might try to teach them some of his ninja ways, but this could lead to misunderstandings and more 'destruction' as they try to adapt his skills to their world.
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.