Well, in 'perospero older brother oc fanfic', a frequent theme is the exploration of his character growth. As an older brother, he may face challenges that force him to become a better person or a stronger leader. Also, themes of sacrifice might be present. He might have to sacrifice his own desires or safety for the sake of his family or the original character in the story. There could also be themes related to the clash between different ideologies within the Big Mom Pirates crew, which he is a part of.
One common theme could be family loyalty. Since Perospero is an older brother, stories might explore how he protects and guides his younger siblings in the context of an original character's story. Another theme could be power dynamics, especially considering Perospero's abilities in the One Piece world.
Common themes in 'perospero older brother oc fanfic' often revolve around his role within the family structure. In many cases, the fic might focus on his relationship with his mother, Big Mom, and how that affects his interactions with his siblings. Additionally, themes of redemption could be present. Maybe he has made mistakes in the past, and through his relationship with the original character, he tries to make amends. Another theme could be the discovery of new powers or abilities, either for himself or for the original character, and how that changes the power balance within the story.