He might be extremely angry and feel a deep sense of hurt. After all he has done for Poseidon and the gods, being betrayed would shatter his trust. He could go into a period of self - exile, distancing himself from the demigod world and trying to find his own path without the help of his father.
One possible reason could be power politics among the gods. Poseidon might see Percy as a liability in some complex power play. Maybe other gods are pressuring him to distance himself from Percy for some hidden agenda. Another reason could be a misunderstanding. Percy might have unknowingly done something that Poseidon misinterprets as a threat to his domain or power. For example, Percy's actions in the mortal world might have unintended consequences for the underwater realm.
Percy could initially feel shock and disbelief. He might try to talk to Poseidon to find out why he was betrayed. If that doesn't work, he could become angry and lash out, perhaps even challenging Poseidon directly.
The allure lies in the fact that Percy already has a very interesting backstory. As the Champion of Poseidon, fanfiction writers can expand on his powers and his place in the mythological world. For instance, he could be sent on quests that are specifically related to Poseidon's domain, like protecting a sacred sea temple from an ancient evil.
In fanfiction, their relationship could be developed by having Poseidon be more involved in Percy's daily life. Instead of just popping in during big events, he could be there for the small things too. For instance, helping Percy with his schoolwork that involves marine biology. They could also have a project together like building a new safe haven for sea nymphs. Through this, they would bond over creating something positive. Also, Percy could start to take on some of Poseidon's responsibilities in the ocean, with Poseidon guiding him. This would not only strengthen their bond but also show Percy growing into his role as Poseidon's son.
Fanfictions may also depict their bonding in the context of quests. Poseidon could be subtly helping Percy from afar during his adventures. He might send sea creatures to assist Percy or give him signs in the form of ocean - related phenomena. This kind of silent support shows their connection, as Percy starts to trust that his father has his back, no matter what.
There's also a fanfic called 'Poseidon's Promise'. In this one, Percy has to deal with a mysterious prophecy related to his father. The story is full of twists and turns. It delves deeper into the relationship between Percy and Poseidon, which is really interesting to read about. It also has some great action scenes with Percy using his water - based powers.
In fanfiction, Percy could be stronger due to the fact that he represents a new generation. He combines the powers he inherits from Poseidon with his own human ingenuity and modern - day knowledge. This combination gives him an edge. Also, fanfiction writers often like to play with the idea of the underdog surpassing the powerful. Percy, being a demigod, is still in a way an underdog compared to a major god like Poseidon, and making him stronger creates an interesting and unexpected twist.
One idea could be Percy feeling betrayed by Poseidon and then finding a new family among other outcasts in the demigod world. He could form a group with them and go on adventures that are not related to Poseidon at all.
One great fanfiction idea could be a story where Zeus, Poseidon and Hades have to team up to face a common threat that is even more powerful than them. Maybe a primordial being awakens and they must put aside their differences. In this story, Percy could be the mediator between them, using his unique position as the son of Poseidon to try and keep the peace among the three powerful gods.