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 could also write a story where the gods from both universes start to merge due to some dark magic. Percy has to figure out how to stop it, because it's causing chaos in his world. He teams up with some of the more sympathetic characters from the God of War universe, like Atreus. Together, they search for the source of the magic and find out that it was a plot by a rogue god who wants to rule over both universes. They then have to fight this powerful god and restore the balance between the two worlds.
One idea could be Percy Jackson teaming up with Kratos from God of War. They could face a common enemy that threatens both the Greek and Norse worlds. Maybe a powerful Titan who has found a way to break free from Tartarus and is causing chaos everywhere.
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 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.
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 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 idea could be Percy having to protect a group of special children who have unique powers related to the Greek gods. Maybe they are descendants of minor gods and are being hunted by some ancient monsters. Another plot could involve Percy teaching these children about their heritage and how to control their powers in a world that doesn't fully understand them.
Perhaps Percy as the God of Justice has to judge the actions of the Olympian gods themselves. This would create a lot of tension as he tries to hold them accountable for their past misdeeds. Or he could be in charge of a special court for demigods who have committed crimes but have valid reasons for their actions, and he has to balance justice and mercy.
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.
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.