One plot idea could be that Gretel and Rock are forced to team up during a dangerous mission in the criminal underworld of Black Lagoon. They have to rely on each other's skills, with Gretel's combat abilities and Rock's strategic thinking to get out alive.
Perhaps a plot where Rock discovers a hidden past of Gretel that changes their relationship. For example, Gretel has a connection to a powerful criminal organization that Rock has been investigating. This could lead to a lot of tension and drama as they try to figure out how to deal with this new information.
One idea could be exploring their past before they became the way they are in Black Lagoon. Maybe a story about their life in the facility where they were experimented on, and how they first started to form their unique bond.
A fanfiction could explore a scenario where Rock and Balalaika are stranded together in a remote location after a deal goes wrong. They have to rely on each other to survive, which is a huge change from their normal relationship in the show. As they struggle, they start to understand each other on a deeper level.
One idea could be exploring the backstories of the mercenaries in more detail. For example, delving into how Revy became such a skilled fighter. Another could be creating a new mission for them that involves a powerful and mysterious enemy. Maybe a shadowy organization that has ties to their pasts.
Perhaps a plot where Balalaika is in trouble and Rock, using his unique skills and knowledge, tries to rescue her. This could lead to a deeper bond between them as they fight their way out of difficult situations together. In the process, Rock might learn more about Balalaika's past and what made her who she is today.
Perhaps a fanfic where Rock and Revy get stranded on an island during one of their smuggling runs gone wrong. They have to rely on each other to survive, and in the process, they start to understand each other better. Revy learns more about Rock's ideals and Rock gets a deeper insight into Revy's survival instincts.
Another plot could involve Rock and Revy getting stranded on an uncharted island during one of their jobs. They must rely on each other to survive. Rock uses his wits to find food and water sources, while Revy protects them from any threats. This situation could also lead to a deeper understanding between them.
Gretel could be developed by exploring her softer side. Usually she is tough and combat - ready, but in fanfiction, we could see her open up to Rock about her dreams or fears. This would add more depth to her character and also change the dynamic between them.
Perhaps a fanfiction could focus on Shenhua's past. Show how she became the skilled fighter she is in the present. It could involve her training under a strict master in a remote location and the events that led her to the Black Lagoon area.
Another plot could involve Balalaika having to protect a valuable artifact that has some connection to her past. This could draw her into a conflict with other powerful factions in Roanapur, all while she is haunted by the memories associated with the artifact. Maybe it was something she lost during a war and now she has a chance to regain it, but at great risk.
Well, a lot of fanfiction might focus on the family dynamics of the Rock Parents. They might struggle to raise their children in such a dangerous and lawless place. There could be stories where they teach their kids important values despite the harsh environment, or how they balance their parental duties with their own survival in the Black Lagoon.