One idea could be that Big Brother is a protector in the BioShock world, guiding Little Sister through the dangerous and strange underwater city. He helps her avoid the Splicers and find the ADAM she needs in a more ethical way, like by using his combat skills to take out the Big Daddies first so she can harvest safely.
A fanfiction could explore the backstories of Big Brother and Big Sister. What were their lives like before they became these iconic figures in Bioshock? You could write about their families, their dreams, and how they ended up in the situation they are in. For example, perhaps Big Brother was once a normal citizen with a family, but the chaos in Rapture forced him into this role. And Big Sister could have a similar yet different origin story.
The plot could revolve around a power struggle between them. They both have their own strengths and areas of control in Rapture. Maybe Big Brother decides he wants more power and tries to take over Big Sister's territory. This leads to a series of battles and strategic moves as they each try to outwit the other.
The Big Brother in Bioshock fanfic is often characterized as strong and fearless. He has a sense of duty towards the Little Sister. For the Little Sister, she is usually depicted as innocent yet with a special power. Her naivete can be both endearing and a liability. For instance, she may not fully understand the danger she's in, which makes the Big Brother's role even more crucial.
A great fanfiction idea could be a Little Sister's rebellion against the established order in Rapture. She starts to question why she has to harvest ADAM and decides to fight against those who control her. This could involve her teaming up with other splicers who also want to change the system in Rapture.
One idea could be that the OC is a protector of the Little Sisters. He or she tries to shield them from the dangers of Rapture while also trying to understand their unique bond with the Big Daddies. Maybe the OC discovers a way to break the cycle of the Little Sisters' harvesting without harming them.
For Big Brother, you can develop his character by showing his past traumas. Maybe he was a survivor of a Splicer attack and that made him determined to protect the Little Sister. His skills can also be developed over time, like learning new combat moves to better fend off threats.
The Big Daddy can symbolize brute force and unwavering loyalty in fanfiction. He is this hulking, armored figure that will stop at nothing to protect the Little Sister. The Little Sister, in turn, can symbolize innocence lost. She is a child, yet she is involved in the dark and gory business of ADAM harvesting. Their relationship symbolizes the co - existence of protection and exploitation in the world of Bioshock.
One interesting aspect could be the exploration of the Little Sister's character in new and different scenarios. In fanfiction, authors might give her more agency or explore her relationships with other characters in ways not fully shown in the original game. For example, her relationship with the Big Daddy could be delved into more deeply, perhaps showing a more emotional bond or her perspective on their symbiotic relationship.
One idea could be that a character from another universe gets transported into the BioShock world and has to team up with a Big Daddy. Maybe they start off as enemies, but as they face common threats like the Splicers, they form an unlikely alliance. The Big Daddy could protect the new character while they figure out how to get back to their own world.
Well, perhaps Naruto could find himself in the world of Bioshock. He might initially be confused by the strange environment and the concept of Big Brother. Maybe he would start using his ninja skills to fight against the oppressive forces that Big Brother represents, all while trying to understand the new rules of this world.