Well, sometimes in fanfiction, his brother might be a mortal who discovers his demigod heritage later. This could add an interesting twist to the story as he has to adapt to the world of gods and monsters. Or it could be a long - lost half - brother from a different camp, like a son of Poseidon from another part of the world who was hidden away for some reason.
A group of rogue demigods could also work well as antagonists. They might be jealous of Percy and his brother's abilities or have their own twisted ideas about how the demigod world should be run. They could create all sorts of trouble for the brothers, from stealing their quests to setting traps for them.
His powers would likely become more god - like. He might gain the ability to control lightning, like Zeus. Also, his strength and speed could be enhanced to a much greater degree. His control over water, which was his main power before, could also be amplified and perhaps combined with his new lightning powers in some unique ways.
His water - based powers might be more intense. Since he's directly related to Poseidon, he could have greater control over large bodies of water, like being able to create huge tidal waves with just a thought, instead of just small ones. He might also be able to communicate with sea creatures on a deeper level, understanding their complex languages and using them to his advantage in ways he couldn't before.
Maybe they embark on a journey to rescue a captured demigod. Along the way, they face various challenges like crossing a dangerous river filled with sea serpents. Percy's knowledge of the water and his brother's problem - solving skills (again, depending on how the fanfiction portrays him) would help them get through the obstacles.
Perhaps they go on a quest to find a lost artifact that has the power to save their mother from a mysterious illness. Along the way, they face various challenges and learn more about their family's past. They might also encounter other demigods who either help or hinder them in their search.
There's a fanfiction called 'Brotherhood in Camp Half - Blood'. In this story, Percy's new brother is introduced in a very dramatic way. The author does a great job of showing how the brother fits into the world of Percy Jackson, with lots of action and adventure sequences. It also explores the relationship between the two brothers in depth, like how they deal with the challenges together and support each other in the face of danger.
In many fanfictions where Percy Jackson is Nico's brother, it creates a whole new dynamic in the story. It often leads to different interactions with other characters. For example, their relationship with Annabeth might change. Percy could be more protective of Nico, and Nico might feel more secure having Percy as a brother. It also gives room for exploring their family history in new ways, like how they deal with their demigod powers together and support each other in the face of various challenges in the demigod world.
There could be. Fanfiction writers are very creative and come up with all kinds of ideas. Since Percy Jackson is a popular character, there are many different takes on his adventures and experiences in fan - created stories. While peeing his pants might seem a bit odd, it could be part of a comical or unique story within the fanfiction world.
His relationship with Annabeth would definitely be affected. She might be a bit wary at first, since Nyx is such an unknown and powerful entity. But she would also be there to support Percy as he figures things out.