In the Percy Jackson universe, it could be Chaos. Chaos is often considered one of the most ancient and powerful entities. If Chaos were the eldest god in the fanfiction, it would bring a whole new level of power and mystery. Percy would be dealing with a being that is essentially the origin of everything, and its motives would be very difficult to understand.
Perhaps it could be Erebus, the personification of darkness. He is one of the primordial deities. If he is the eldest god in the story, Percy would be facing a being that is associated with the very essence of the unknown and the hidden. This could lead to some really dark and suspenseful plot developments as Percy tries to figure out Erebus' intentions and how to deal with his power.
Another possibility could be Gaia. She is an extremely old and powerful goddess in Greek mythology. If she was portrayed as the eldest god in the fanfiction, she could have a complex relationship with Percy. Maybe she has some grand plan to reshape the world and Percy is an unexpected factor in her plans, either as an obstacle or a potential ally depending on how things unfold.
Nyx, the goddess of the night, is quite popular in fanfictions. Her dark and mysterious nature makes her an appealing character. Writers can play with her powers, such as her control over the night, dreams, and all the things that lurk in the darkness. She can be portrayed as a powerful and independent figure, or she can have complex relationships with other gods or demigods. Erebus, the god of darkness, is also a favorite. His connection to Nyx and his role in the primordial cosmos offer a lot of material for fanfiction writers. He can be a source of great evil or a misunderstood figure trying to find his place in the world of the story.
Another plotline could involve the eldest god having a prophecy related to Percy. This prophecy could force Percy to make difficult choices between his loyalty to his friends and his newfound role in the eldest god's plans. For instance, he might have to sacrifice a chance to save a friend in order to fulfill a greater purpose for the world as dictated by the eldest god.
A frequent theme is the resurrection or return of the eldest gods. In some fanfictions, the idea that these long - forgotten or sleeping gods awaken to find a world that has changed drastically is explored. This can lead to all kinds of conflicts and reconciliations. Also, the concept of the eldest gods' domains is a big part of it. For instance, if a fanfiction focuses on Gaea, the earth goddess, it might explore how she views the modern world's treatment of the earth. And the theme of redemption can also be found. Maybe an eldest god has made mistakes in the past and now tries to make amends in the context of the Percy Jackson universe.
He might be very wise. Having been around the longest, he would have witnessed countless events and learned from them. So, he could offer very profound advice to other characters in the fanfiction.
It could be Ares. He is often portrayed as a violent and unpredictable god. Percy might banish him if he goes too far in causing wars and chaos among mortals. His banishment would have a huge impact on the battlefields of the mortal world.
Apollo might be the betrayed god. He could have been tricked by another god into betraying Percy. For example, Apollo might have been promised more power or a higher status among the gods if he turned against Percy. But then he realizes too late that he's been used and that he's actually betrayed someone who could have been an ally.
Poseidon is a likely candidate. Since he is Percy's father in the original series, it would make sense for him to take a more active role in raising Percy. He could teach Percy all about the ocean, its secrets, and how to use his water - related powers more effectively.
It could be a new, powerful Titan that has emerged from the depths of Tartarus. This Titan might be jealous of the eldest primordial's power and wants to either destroy it or take its power for itself. And since Percy is associated with the eldest primordial, he becomes the Titan's target as well.
His mother, Sally, would surely be one of his allies. She has always been there for him. Also, Grover could be his ally as they have a long - standing friendship. Grover might see Percy as an even greater protector of the nature spirits now that he's a god.
Some minor gods related to the earth like Demeter could be allies. She has her own powers over the earth and could teach Percy some of her techniques. Also, the nymphs, such as the water nymphs and the tree nymphs, could be allies. The water nymphs could help with the water sources within the earth (like underground rivers) and the tree nymphs could help Percy take care of the forests. And let's not forget the pegasi. They could be used for transportation when Percy needs to quickly travel across his earthly domain.