There could be a fallen angel who is bitter towards God. In the context of the Hazbin Hotel fanfic, this fallen angel could try to use the chaos and desperation of the sinners in the hotel to his advantage. He might try to turn the sinners against the idea of redemption and God, causing all sorts of problems for the hotel and its staff who are trying to promote the path of redemption.
Charlie would definitely be a main character. Her whole goal of redeeming sinners in the Hazbin Hotel could be greatly affected by the presence or actions of God. Also, Vaggie, as Charlie's girlfriend, would be involved in any major changes that come about. And Lucifer could be a key character too, as his relationship with God is a central part of the lore, and his reaction to God's possible interference in the hotel would be interesting to explore.
One idea could be that God decides to visit the Hazbin Hotel incognito to see if redemption is truly possible in this unique afterlife setting. He could interact with the characters like Charlie and Vaggie, observing their efforts and maybe secretly guiding them in subtle ways.
A powerful mythological creature that has a grudge against the god Percy is championing for. For example, a Titan who was once defeated by that god and now sees Percy as a way to get revenge. It would be a great adversary, with its own set of powers and minions.
Charlie is one of the main characters. She is the princess of Hell and the one who runs the Hazbin Hotel with the goal of rehabilitating sinners. Angel Dust is also a major character, a flamboyant and complex sinner. Then there's Vaggie, Charlie's girlfriend, who is very protective of Charlie and the hotel.
Alastor is also very popular. His radio - host persona and his rather mysterious and powerful nature draw a lot of attention. Fanfic writers like to explore his past, what made him the way he is, and his relationship with the other characters in the hotel.
Angel Dust is another popular character. His flamboyant personality and complex backstory mean that his reactions in the fanfic can be both humorous and deeply emotional. Viewers are often drawn to his character arc and how he'll grow or change in response to different situations.
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.
A powerful Titan who wants to use Percy's newfound power as the God of Pain for his own evil purposes could be the main antagonist. This Titan might see Percy as a tool to spread chaos and destruction, and he tries to manipulate Percy into using his pain - based powers to harm innocents. Percy would then have to resist this Titan's influence and fight against him.
Perhaps a forgotten or banished god could be the main antagonist. This god, filled with resentment for being cast out, could return with a vengeance and start causing chaos. He might be powerful enough that the only solution Percy sees is to slay him. This antagonist could bring in a lot of new lore and backstory to the fanfic as Percy uncovers the secrets of this god's past.
Perhaps Orochimaru from Naruto could be the main antagonist. He's always seeking more power, and in this fanfic, he might see the powers in 'The God of High School' as a means to achieve ultimate power. He could try to manipulate the events of the 'The God of High School' tournaments to his advantage, all while causing chaos in Naruto's world as well.