In some fanfictions of this kind, the brother could be from a different background or have a hidden past. This could add an element of mystery to the story. He might bring in new allies or enemies because of his past. And as he steps into Percy's shoes, he has to deal with the expectations that were set by Percy, while also trying to be true to himself.
One possible plotline could be that the brother has different abilities or a distinct personality. Maybe he approaches the same challenges Percy faced in a completely new way. For example, if Percy was known for his courage in battles, the brother might use his intelligence to outwit the enemies instead.
Obviously, Percy's brother is one of the main characters. He is the one who is replacing Percy. And the characters who were associated with Percy like his friends, family members, and maybe some of his foes are also likely to be main characters as they would be affected by this replacement and would interact with the brother.
Another interesting plot could be Percy's internal struggle. He's used to being Poseidon's son, and now this new connection to Styx throws his identity into a bit of chaos. He has to figure out how to balance his old self with this new aspect of his being, all while facing various quests and threats in the Percy Jackson universe.
Another plotline might involve family drama. Since they are brothers in this fanfiction, there could be conflicts over their different approaches to things. For example, Luke might be more reckless while Percy is more cautious, leading to arguments between them. And as they grow, they have to learn to understand and accept each other's differences.
One plotline could be the overprotective brother constantly trying to keep Percy out of dangerous quests. He might try to interfere with Chiron's plans for Percy, thinking he's too young or inexperienced. Another could be the brother getting into arguments with Percy's friends like Annabeth when he feels they're leading Percy into harm's way. And there could also be a plot where the brother himself gets involved in a quest to protect Percy and discovers his own hidden powers along the way.
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.
One possible plotline could be Percy's struggle with his new godly powers and how it affects his relationships. For example, his friends might be in awe or a bit intimidated by his new status.
One possible plotline could be Percy having to adjust to the unique rules and traditions of the hunters. For example, dealing with their vow of celibacy in a humorous or challenging way. Another could be Percy going on a special hunt with the hunters that involves a mythical creature not commonly associated with them.
One possible plotline could be their adventures in a new, undiscovered part of the mythological world. Maybe they stumble upon a hidden realm full of ancient and powerful creatures that they have to either befriend or fight.
Well, perhaps in this 'percy is an orphan fanfiction', there's a plotline where Percy is constantly bullied because he's an orphan. But then he discovers a hidden power within himself that not only helps him stand up to the bullies but also starts to unravel a mystery related to his family. Maybe he finds out that his parents were part of a secret organization and their disappearance was not accidental.
One interesting plotline could be them teaming up to rescue a captured demigod. They could use their unique abilities, Percy's water - related powers and Bianca's skills with a certain weapon or magic. Another plotline could be them exploring an unknown part of the mythological world together, facing strange creatures and ancient puzzles.