Percy might initially be an outcast among the other demigods. His parentage is quite different from the normal Olympian - related demigods. They might be wary of him because they don't understand his powers. However, some brave demigods could be curious and try to befriend him.
Perhaps Percy discovers a prophecy related to his birth. It could state that he is the key to either saving or destroying the world. This would create a lot of internal conflict for him. On one hand, he has his friends and the people he cares about in the mortal world. On the other hand, his powers and his role according to the prophecy could force him to make some very difficult choices.
Well, it could be about Percy having to deal with the powers and heritage that come with being the adopted son of Pontus. Maybe he discovers new underwater abilities that are different from his Poseidon - given powers. He might also have to face enemies who are after the secrets Pontus has.
Perhaps an ancient sea god who was once an enemy of Pontus. This god could see Percy as a threat because of his heritage. He would use his own minions, like sea serpents and murlocs, to attack Percy and those he cares about. He might also try to corrupt Percy, tempting him with power to turn against his friends.
The plot could involve Percy's journey to discover the true meaning of being the son of Gaea and Pontus. He could be on a quest to find out more about his parents' realms and how they interact with the world of the demigods. Along the way, he may encounter mythical creatures that are related to Gaea and Pontus, and he has to either fight them or make peace with them. There could also be a sub - plot about Percy's relationships with his friends at Camp Half - Blood, as they struggle to accept this new side of him.
Well, in fanfiction, if Percy is the Champion of Pontus, his story would be filled with more mystery. Pontus is an ancient and powerful entity. Percy might be called upon to do things that go against the traditional ways of the Olympian - influenced world he's used to. He could discover secrets about the origin of the sea and his own powers that were previously unknown. This would not only make for an exciting read but also show how Percy can adapt and make his own path even when given a very different role.
Developing his powers further is also a way. Since he's the son of Nyx, he could learn to manipulate different aspects of darkness like creating illusions using shadows or traveling through the dark realms faster. This would not only make him more powerful but also show his growth as a character as he masters these new abilities.
The Titans who were once associated with Pontus might be antagonists. They could see Percy as a threat to their plans if he is related to Pontus. They might try to stop him from fulfilling any tasks given to him by Pontus or try to use his new identity against him.
One idea could be Percy discovering new powers related to being the son of Pontus. For example, he might be able to control underwater currents in ways he couldn't before. Another plot could involve him having to face a new type of sea monster that is specifically drawn to his Pontus heritage. He could also go on a quest to find an ancient artifact related to Pontus that has the power to either save or destroy the underwater world.
The other gods might be antagonists. Since Percy is the son of Gaea and Pontus, the Olympian gods could see him as a threat. They might be worried that he'll side with his primordial parents and disrupt the balance they've been trying to maintain. So, they could send their demigod children or even some minor gods to oppose Percy.