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.
One common theme is the struggle with identity. Since Percy is the son of Erebus and Nyx in these fanfictions, he often has to grapple with being different from the 'canon' Percy. Another theme is power. Given his new parentage, he might have different or enhanced powers, and the fanfictions often explore how he uses or controls them.
The Titans could be antagonists. Since Nyx is an ancient and powerful goddess, the Titans might see Percy as a threat because of his connection to her. They could try to either turn him against Nyx or simply eliminate him.
Another plotline could involve Percy exploring the powers he has inherited from Chaos and Nyx. Maybe he discovers unique abilities that no one has ever seen before, like the power to manipulate shadows in a way that's different from a child of Hades. This could lead to him going on a journey to understand the full extent of his powers and how to control them.
One idea could be Percy having to face a new type of monster that is related to Nyx's powers. For example, a shadow - creature that can only be defeated by using the unique abilities he inherits from being the son of Nyx.
One idea could be Percy's struggle with his new identity as Nyx's adopted son. He might face discrimination from some of the other demigods who are suspicious of his new connection. Another plot could involve Percy learning new powers from Nyx that set him apart in battles against monsters. He could also have to deal with Nyx's enemies who target him because of his relation to her.
Well, obviously Percy himself. Since he is the son of Chaos and Nyx in this fanfiction, he is the central figure. Annabeth is often included too. Her relationship with Percy, whether it's evolving or being tested due to his new status, is a key aspect. There could also be other demigods like Grover. He might be there to support Percy or be a bit wary of his new powers.
A plot twist could be that Percy has a connection to the primordial forces that even the Titans and the Olympians are afraid of. His link to Nyx and Tartarus means he can access parts of the universe that are normally off - limits. For instance, he might be able to open portals to the darkest corners of Tartarus, which could either be a great advantage or a huge danger. Also, he could have the power to call upon the creatures of the night created by Nyx, and these creatures could be either his allies or a threat to the existing order of the demigod world, depending on how Percy chooses to use his powers.
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.