Imagine a time - traveling demigod from the future. This character comes back to warn Percy about a great disaster that will happen if he doesn't change certain events in the present. He could bring with him new technology or knowledge from the future that both helps and complicates Percy's situation.
A character could be a young oracle who has visions related to Percy's new adventures. She could be a bit of an enigma, not fully understanding her own powers and sometimes giving cryptic messages that Percy has to decipher. This would add an element of mystery to the story.
A new character could be a half - angel, half - mortal who becomes Percy's ally or rival. They have powers related to both the Christian concept of angels and the Greek demigod powers. Another could be a seer from an ancient, long - forgotten civilization who has knowledge about Percy's true potential as a son of god.
There could be a character who is a mentor to Percy in understanding his devilish side. This character might be an ancient being with knowledge of both the divine and the demonic. For example, a fallen angel who has seen the rise and fall of many with such mixed heritages.
There could be a new oracle who is specifically attuned to Percy's 'son of fate' situation. This oracle might be a young demigod with unique powers that help Percy understand his fate better. Maybe they can see visions of the future that are directly related to Percy's journey.
Perhaps some minor gods who have been overlooked in the past. They could see Percy's new status as the 'Son of the Universe' as an opportunity to gain more influence and decide to ally with him. For example, the god of small rivers or the goddess of wildflowers.
There could be ancient Greek heroes that Percy meets in the past. For instance, Heracles. Since Percy is from a different time, Heracles might not be exactly as the legends say, and they could have an interesting dynamic. Another potential character could be an oracle from an older era who has different prophecies related to Percy's situation.
A young demigod who has the power to communicate with the spirits of the monsters. This character could help Percy understand the motives of the monsters better and maybe find non - violent solutions to some of the conflicts.
One potential new character could be a selkie. Selkies are mythological creatures that can transform between seal and human form. This selkie could be an ally or a foe to Percy. Maybe there's a sea nymph who has a special connection to Percy's mother. She could show up to help Percy when he's in a tight spot. And what about a sea giant who was once an enemy of Neptune but now wants to make amends? He could be a very interesting addition to the story.
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.
Obviously, Percy Jackson himself. And Annabeth could also be there, helping Percy understand the new environment. There might be some new characters like an Alaskan native who is a seer and can guide Percy in his adventures.
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.