One consequence could be that he might isolate himself from his friends. In fanfiction, Percy is often depicted as a social and heroic figure. Losing hope could make him feel like he's a burden to others and cause him to withdraw. For example, he might stop showing up at Camp Half - Blood activities.
It's possible that his powers could even be enhanced in some ways. Losing an eye could make him more in tune with his other senses, and this could translate to his powers. For instance, his ability to sense danger through water vibrations could become sharper as he has to rely more on it. His powers might also develop in new directions. Maybe he could start to use water to create illusions to compensate for his lost vision in battle, like using water to project false images of himself to confuse his enemies.
Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
There's also 'Female Percy: A New Heroine'. Here, Female Percy brings a fresh perspective to the story. Her fighting style is different, more fluid perhaps. And her relationships with the gods are also unique. Instead of the typical male bravado, she uses her wits and charm to get through tough situations.
Arthur could be the cause. He often bosses Merlin around and doesn't fully understand Merlin's capabilities. His constant demands and lack of respect for Merlin's intelligence might push Merlin to the edge. For instance, when Arthur makes Merlin do dangerous and pointless tasks without any regard for his safety.
A popular one is 'Percy and Annabeth's New Quest'. In this fanfiction, the author takes the duo on a quest that wasn't in the original books. They have to solve ancient riddles and battle new mythical creatures. It stays true to the characters' personalities, making it a great read.
You can start by looking on fanfiction.net. It has a large collection of 'Percy Jackson' fanfictions, including many that focus on Percy and Piper. Just use the search bar and type in relevant keywords like 'Percy and Piper' and you'll get a bunch of results.
One great 'Percy Jackson, Percy and Piper' fanfiction is 'The Adventures of Percy and Piper'. It really captures the essence of their characters. The author does an amazing job of creating new quests for them while still staying true to their personalities from the original series.
There are also fanfictions where Percy's mother's death causes him to question his whole existence. He wonders why the gods let such a thing happen to a good person like his mother. This internal struggle could lead him to have some tense interactions with the gods. He might even consider rebelling against them in some way, feeling that they are partly responsible for his mother's death.