There's a chance that it's a new entity created by the imbalance between order and chaos. This entity is a force of pure destruction and wants to tip the scales completely in favor of chaos. Percy, Nico and Jason, with the help of Chaos, have to stop this entity from achieving its goal.
One of the common themes in fanfictions involving Percy, Nico, Jason and Chaos could be their epic adventures. For example, they might team up to face a new and powerful threat that emerges from the chaos itself. Maybe they have to journey through different realms, using their unique powers and personalities to overcome obstacles.
A cool fanfiction idea could be that Percy, Nico, Jason are chosen by Chaos to be its champions in a battle against an ancient evil that is trying to bring total destruction. They are given special powers by Chaos and have to learn to use them quickly.
In this fanfiction, it could be that Percy, Nico and Jason are chosen by Chaos for a special mission. Chaos might have seen something unique in them, like Percy's leadership skills, Nico's connection to the underworld and Jason's strategic mind. They join Chaos and are part of a team that has to go on a quest to find some lost artifacts that are crucial in maintaining the balance between order and chaos in the universe. Along the way, they face many challenges, make new friends and enemies, and grow as characters.
It could be a senior officer who has a grudge against Percy for some reason. Maybe Percy accidentally made him look bad in front of the higher - ups early on.
A powerful monster from the demigod world that has a vendetta against Percy and Nico could be the main antagonist. It disguises itself as a teacher or a student and slowly starts to manipulate things in the school. It tries to isolate Percy and Nico from their friends and then attack them when they are at their weakest.
Perhaps an old enemy from Greek mythology who has found a way to infiltrate the school in a modern form. This could be a minor god or a powerful monster. They want to get revenge on Percy and Nico for past defeats. They might be using the school's resources, like the science labs or the library, to create chaos or find a way to regain their full power.
A great antagonist could be a rogue demigod who has been corrupted by power. This demigod could have discovered a way to combine the darkest parts of Greek and Roman magic and is using it to cause chaos among both camps. He or she might see Jason and Percy as the biggest threats to their plans and target them specifically.
However, Jason has his own set of powerful abilities. His control over lightning gives him a great offensive power, and his ability to fly can give him an advantage in both battle and mobility. In some fanfictions where the setting is more favorable to his powers, he could be considered the most powerful. For example, in a high - altitude battle or in a place where there are a lot of electrical sources he can draw power from.
A powerful chaos deity could be the main antagonist. This deity might see Percy as a threat to their plans of spreading chaos throughout the world. They could constantly send their minions to attack Percy and try to corrupt his mind.
Another possible antagonist could be a rogue demigod who is corrupted by the power of Chaos. This demigod might envy Percy's connection to Chaos and decide to turn against him. He could form his own group of followers, causing chaos (literally) in the demigod world. They would oppose Percy and his friends at every turn, believing that they are the rightful heirs to the power of Chaos.