One possible plot could be that Percy and his friends are faced with a new and extremely powerful threat that endangers not only the mortal world but also the entire pantheon. In order to defeat it, they are offered a chance to become gods. They have to go through a series of trials that test their loyalty, courage, and wisdom even more than before. Percy might struggle with the idea of leaving his mortal life and family behind, but realizes it's for the greater good. Along the way, they face old enemies who are jealous of their potential ascension and try to stop them at all costs.
In this fanfiction, it could start with Percy and his friends being chosen by some ancient force to become gods. As gods, they may have to form a new pantheon. They will need to establish rules and hierarchies within their group. There could be a lot of internal conflict as they each have different ideas about how to use their powers and rule. Some might want to be benevolent and help mortals freely, while others might think that mortals should fend for themselves more. This would lead to a lot of drama and character development within the story.
Well, there could be Percy going on quests with the gods by his side. Maybe they help him face new and more powerful monsters that he couldn't handle alone. For example, fighting a giant that has been awakened by an ancient curse.
Grover might be the most likely to resist. He has always been very connected to the mortal and natural world. His role as a satyr has been to protect and guide demigods in the mortal world. Becoming a god would mean leaving behind his familiar role and the simple joys of the natural world he loves so much. He might fear losing his identity and his close connection to the forests, meadows, and the young demigods he has befriended over the years.
In this fanfiction, it could be that there is a prophecy that Percy is fated to either save or destroy the gods. Circumstances push him towards the latter. He might learn some dark secrets about the gods' origin that make him believe they are a threat to all existence. So he embarks on a journey filled with challenges and battles to take them down.
One possible plot could be that Percy has somehow angered the gods, perhaps by accidentally disrupting a major ceremony. As a result, they send various mythical creatures to hunt him down. He has to use his wits and demigod powers to outsmart them and find a way to make amends with the gods.
In this fanfiction, the plot might revolve around Percy's struggle as a slave of the gods. He could be used as a tool by the gods in their power games. For example, the gods might send him to retrieve powerful artifacts from dangerous places, and all the while he is trying to resist this unjust treatment and regain his freedom.
Athena might also be a good candidate. Although she and Poseidon have their differences, she could see Percy's potential and befriend him. She could offer him wisdom and strategic advice for his various adventures.
Perhaps the fanfiction could center around Percy discovering a hidden prophecy that is the key to ending the war between the gods. He then has to go on a dangerous quest to fulfill it. Along the way, he meets new allies and enemies, and has to use his wits and powers to overcome various obstacles. This could also lead to some character development for Percy as he faces difficult decisions and moral dilemmas.
Maybe Percy unknowingly breaks some ancient and very strict laws of the Gods. For example, he might interfere with some sacred rituals or places that are off - limits to mortals, even demigods like him. In the fanfiction world, the Gods could be very unforgiving when it comes to such violations and decide to kill him as a result.
Maybe Percy hides because he discovers a secret about the gods that they don't want him to know. For example, he might find out about some underhanded deal among the gods that could endanger the demigods. So, in order to protect himself and his friends, he decides to hide.