Well, there could be new demigod children of Hades. Maybe a girl who has the power to control the shadows in a unique way, like she can use them to create portals to different parts of the underworld. Or a boy who can communicate with the dead in a more advanced way than the usual children of Hades, and he has some sort of prophecy related to Percy.
There might also be some new monsters from the underworld. For example, a giant scorpion - like creature that is specifically bred in the darkest corners of Hades' domain. It could be a major obstacle for Percy and the other characters in the fanfiction. And then there could be a minor god or goddess related to Hades' realm, like a deity of forgotten souls who has some sort of connection to Percy's quest or the overall plot.
Annabeth Chase might also be an ally. Her intelligence and strategic thinking could be valuable assets for the son of Hades. She could help him plan his moves, especially when dealing with the complex politics among the demigods and gods.
A centaur who has lived in both the Roman and Greek territories. He could be a mentor to Percy, teaching him about the similarities and differences between the two cultures' ways of combat, magic, and survival. He could also have a lot of stories about the ancient times when the two cultures were more intertwined.
A new demigod from a previously unheard - of god or goddess. Maybe a child of a minor deity who has some unique powers that could either help or hinder Percy in the situation of the collapse. For example, a child of Hypnos who can manipulate sleep and dreams, and their powers could be used to either give Percy visions of how to fix things or put him in a dangerous trance.
In a 'Percy son of Hades fanfiction', there are many characters that could be added. For example, Charon, the ferryman of the Underworld. He could play a role in Percy's travels within the Underworld. Persephone could also be an important character. She might be initially suspicious of Percy but then come to see his potential. Additionally, the Erinyes, or the Furies as they are more commonly known, could be added. They could be a source of conflict or assistance for Percy, depending on the situation.
A group of rogue demigods who have been using vapor travel to steal from other realms. They could be both enemies and potential allies depending on the situation. They might have unique powers related to vapor manipulation that Percy and his friends have never seen before.
A new demigod from a really rare godly parentage, like a child of Hypnos, the god of sleep. This character could have unique powers related to dreams and illusions and could be an ally or an adversary to Percy. Another could be a centaur who has been exiled from his herd for some reason and has a vendetta against the Olympians. He could be a source of knowledge for Percy about some of the more hidden secrets of the gods.
A mermaid - centaur hybrid could be a really cool new character. This character could have the upper body of a centaur and the lower body of a mermaid. They could be an expert in both land and sea magic, and help Percy on his quests. They might also have a mysterious past that is related to the ancient wars between the gods.
Frank Zhang could also be an ally. He is a Roman demigod with shape - shifting abilities. His strength and loyalty would make him a valuable teammate for Percy. Additionally, Nico di Angelo, who has ties to both the Greek and Roman worlds, could be an important ally. His knowledge of the Underworld and his ability to shadow - travel could help Percy in various situations, whether it's getting information or quickly moving around in the Roman camp.
Annabeth would definitely be one of his top allies. She's been with Percy through many adventures and her intelligence would be crucial in helping Percy understand his connection to the creator and any new situations that arise. For example, she could research ancient texts to find clues about the creator's intentions for Percy.
Nico di Angelo could be a great ally. Since he has his own connection to the Underworld, he might understand Percy's new situation better. Another could be Thalia, as she's always been a strong and loyal friend. She would likely stand by Percy despite his new dark powers. Also, Grover could be an ally. His empathy and loyalty would make him support Percy no matter what.