Another idea is that OC has a special ability that Elizabeth discovers can help them control the Big Daddies in a different way. They then use this to their advantage to take on the main antagonists in Rapture, like Sofia Lamb. As they do this, a bond forms between them, and they learn more about each other's pasts and how they ended up in this strange and terrifying place.
Perhaps OC is from the surface world and comes to rescue Elizabeth. Along the way, they have to face the moral dilemmas of the underwater city, like whether to use the power of ADAM or not. This would create a lot of tension and character development as they journey through the dangerous and beautiful but corrupt world of Bioshock.
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.
One idea could be exploring the backstory of a minor character in the Bioshock universe. For example, a splicer's journey before they became spliced. It could show how they were once an ordinary citizen of Rapture and what led them to make the fateful decision to use ADAM. Another plot could focus on an alternate timeline where the protagonist makes different choices. Maybe they side with the wrong faction and have to deal with the consequences. A third idea is a story centered around a new type of plasmid that has unexpected effects on the user and how it impacts the power dynamics in Rapture.
OC could be developed by giving them a unique backstory related to the world of Bioshock. For example, they could be a descendant of an early Rapture scientist who was sent away before the fall. This would give them a connection to the place and a reason to explore it.
One interesting story could be about an original character who has unique powers and how they navigate the complex world of Bioshock Infinite. Maybe they have the ability to control the Songbird in a different way than the main characters in the game.
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.
One idea could be a crossover with another game like "The Elder Scrolls". Imagine Booker DeWitt finding himself in the world of Skyrim. He could use his skills and weapons from BioShock Infinite in a completely different fantasy setting, facing off against dragons and magic - using vigors in a new and unexpected way. Another idea might be a crossover with a science - fiction series like "Mass Effect". How would the plasmids and the genetic manipulation in BioShock Infinite interact with the biotics and advanced technology in Mass Effect? It could lead to some really cool battles and character interactions. Also, a crossover with "Alice in Wonderland" could be fascinating. Picture Elizabeth falling down the rabbit hole into Wonderland. Her ability to open tears could have a really interesting impact on the strange and dream - like world of Wonderland, perhaps creating new areas or changing the rules of the land.
One idea could be exploring how Ciel and Elizabeth handle the news. Maybe Ciel becomes overly protective, trying to keep Elizabeth safe from all possible threats. He could hire extra guards and limit her outings. Elizabeth, on the other hand, might feel a bit stifled but also understand his concern. Another plot could involve how their families react. Elizabeth's family might be overjoyed and start planning a grand celebration, while Ciel's family business and the underworld he's involved in could pose some challenges to the pregnancy, like potential enemies using it as a weakness.
One idea could be a story where Richard III has a change of heart regarding his claim to the throne and decides to support Elizabeth of York's rightful place instead. This could lead to an alliance between them that changes the political landscape.
One idea could be that Jack and Elizabeth are on a quest to find a hidden treasure that is said to have the power to control the seas. Along the way, they face various challenges like sea monsters and rival pirates. They have to rely on each other's skills, Jack's wily ways and Elizabeth's determination.
One idea could be exploring how they adjust to the news. Maybe Mr Darcy becomes overly protective, while Elizabeth is determined to maintain her independence during the pregnancy. They could face challenges from society, like gossip and speculation about their quick family expansion.