Another theme is redemption. Given the events in Bioshock, Elizabeth may try to atone for things that happened. This could involve making up for the actions of others or her own actions that she feels guilty about. It often makes for a very emotional and engaging read.
A really interesting one is 'Elizabeth's Redemption'. In this fanfic, Elizabeth tries to make up for some of the chaos that was caused in the Bioshock universe. It's a deep exploration of her guilt and her journey to find peace and do better, with some really emotional moments.
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.
Yes, there are likely many fanfictions that focus on the relationship between Elizabeth and the scientist in a non - sexual way. These can explore their partnership, their shared adventures in the Bioshock universe, or their character development. You can start by looking on fanfiction - specific websites and filtering for appropriate content.
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 possible storyline could be them working together to save the city from a new threat. Maybe they discover a new type of enemy or a dangerous technology that could destroy their world, and they have to use their combined skills to stop it.
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.