Perhaps an ancient enemy of the gods that was sealed away in time. Percy's time - travel disrupts the seal, and this enemy is released. It could be a primordial being with powers beyond what the modern gods and demigods are used to. This being would want to destroy the current order of things and reshape the world according to its own dark desires, using time - travel - related anomalies to its advantage.
A Kronos - like figure could be a likely villain. If someone were to time - travel and accidentally free a younger, more malevolent version of Kronos, he could cause all sorts of chaos. He has a grudge against the Olympians and would stop at nothing to regain his power.
It could be a rogue god. Maybe a god who has been forgotten over time and feels resentful. This god, through time - travel, could try to rewrite history to make themselves more powerful. They could manipulate events in the past to change the power balance among the gods in the present. For example, a minor god of chaos who decides to use Percy's time - travel as an opportunity to start a new era of disorder.
Circe is another option. She has the power to transform people and cause chaos. In the fanfiction, she could be jealous of Wonder Woman's beauty and power and Percy's connection to the gods. She might try to turn them against each other by using her magic on them or on those around them, causing misunderstandings and battles.
His mother, Sally Jackson, could play an important role as an ally. She has a strong will and has always been there for Percy. Also, Tyson, his cyclops half - brother, could be a powerful ally with his strength and loyalty. And don't forget about the Nymphs. Some of the water nymphs might help Percy since he has a connection to water.
The main villain could be an ancient Titan who was imprisoned long ago but has now found a way to break free. This Titan could be seeking revenge on the gods and demigods alike, and Percy and his friends would have to stop him before he destroys the world as they know it.
It could be Luke. Even in a time - travel scenario, Luke's bitterness towards the gods might lead him to betray Percy. He could use time - travel to his advantage, perhaps making deals with past versions of enemies to stop Percy from achieving something that would benefit the gods.
Percy Jackson himself would be the main character, obviously. Since he is the God of Matter in this fanfiction. His friends like Annabeth Chase would also likely be important characters. She could be there to support Percy and use her own intelligence to help him understand and control his new powers.
There could be new characters introduced. Maybe a being from the void itself, a sort of anti - hero or a misunderstood creature that Percy has to work with. Also, some of the old Gods like Poseidon might play a role. Poseidon could be either worried about his son's new status or trying to use it to his advantage in the complex politics of the Gods.
Obviously, Percy Jackson himself would be a main character. Since it's about him being in a god camp. Annabeth Chase is also likely to be a main character as she has always been an important part of Percy's life. Grover, Percy's best friend, would probably be there too. And there might be some new characters, like other god - like beings or powerful demigods in the camp that Percy interacts with.
In Percy Jackson fanfiction, Zeus is often considered one of the most powerful gods. He is the king of the gods, controls the sky and lightning. His power is so great that it can shake the entire world when he's angry. Just think about the times in the original stories when he showed his might, and in fanfiction, that power is often amplified even more.
One likely opponent is Luke Castellan. Since they have a complex relationship in the original series, it would be a very emotional and intense battle in fanfiction. Luke has his own set of skills and powers, and his motives would make the battle even more interesting.
Kronos could make a comeback. Even though he was defeated before, he could find a way to return in a more powerful and vengeful form. He could be the mastermind behind the apocalypse, seeking to finally destroy the gods and take over the world once and for all.