One main challenge is blending the two very different settings. Bioshock has a dark, industrial, and often claustrophobic setting, while RWBY has a more colorful and expansive fantasy world. Another is making the characters' powers and abilities work in the new context. For example, Ruby's scythe skills need to be adjusted when facing Bioshock's enemies. Also, the different tones of the two - Bioshock's dark and often disturbing tone vs RWBY's more hopeful and action - adventure tone - need to be reconciled.
One great 'Bioshock and RWBY Crossover Fanfiction' is 'A New World'. It combines the unique aesthetics of RWBY with the dark and dystopian atmosphere of Bioshock. The characters from RWBY are thrown into the world of Bioshock and have to adapt to survive. Another one could be 'Merged Realms' which explores how the powers in RWBY would interact with the technology and threats in Bioshock.
The 'Bioshock Graphic Novel' is significant as it expands the Bioshock universe. It gives more backstory to the world of Rapture. For example, it might show how certain characters came to be in Rapture or what events led to its decline in a more visual and narrative - driven way compared to just the game.
It expands the story. It gives more background on the world and characters of Bioshock Infinite. It can show things that might not be as easily conveyed in the game, like the early lives of the main characters or the history of the floating city.
Redemption can also be a theme. Maybe the Big Sisters are seen as more than just the menacing figures they initially seem. The OC could play a role in redeeming or understanding them better. In some stories, the OC might discover a more human side to the Big Sisters and try to help them break free from their programming. This can create a really interesting dynamic in the 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.
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.