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.
Well, it could be that Percy is raised by a specific god like Poseidon but in a more direct and involved way. Maybe the god teaches him all kinds of super - secret powers and knowledge that normal demigods don't get. For example, he might be taught how to control water in ways that are far beyond what he learned at Camp Half - Blood. This could lead to him having a whole new set of adventures, facing different kinds of monsters and challenges that are related to the god's domain.
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.
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.
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.
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.
Well, it really depends on the specific fanfiction. There are countless fanfictions out there about Percy Jackson. Some authors might choose to elevate him to god status as a way to explore different aspects of his character, like how he would handle immortality or the responsibilities that come with being a god. They could also do it to create interesting new power dynamics within the Percy Jackson universe.
His friends from Camp Half - Blood like Annabeth could be his allies. Annabeth is smart and strategic, and she would be able to offer valuable advice as Percy goes through the process of becoming a minor god.
The Cyclops Tyson could be an ally. He has a strong bond with Percy and his strength could be a great asset. In a fanfiction where Percy is the God of Light, Tyson could be like a protector of Percy's light - based temples or strongholds. Also, the sea creatures that Percy has befriended over the years might come to his aid. Since Percy has his roots in the sea and now has light powers, they could work together in a unique way, like the sea creatures using their natural abilities to help spread Percy's light across the oceans to keep dark sea - based threats at bay.
It could be an old enemy who now sees Percy as an even greater threat. Maybe Kronos' remnants. Since Percy is now a god, they might fear that he'll be able to fully destroy them. So they try to stop him from getting used to his powers or try to steal his powers.