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.
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.
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 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.
One idea could be that the creature forms an unexpected alliance with a local scientist who wants to protect the lagoon from being exploited. They work together to fend off greedy developers.
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.
Another plot could involve the two universes being on a collision course because of some interdimensional rift. Ghost Rider and the main characters of Black Lagoon must work together to close the rift while also dealing with the threats that come from both worlds merging. There could be a lot of action and character development as they learn to trust and rely on each other in this strange situation.
One idea could be that Naruto and his team are sent on a mission to the world of Black Lagoon. They team up with Rock to take on a powerful crime syndicate that is after a powerful jutsu Naruto has. Another could be that Revy gets transported to the Naruto universe and has to deal with the shinobi world's politics while also teaching some of the younger ninjas her combat skills.
First, focus on her personality traits like her independence and her combat skills. You can start with a situation that challenges these traits, for example, if she's captured and has to rely on her wits rather than just her fighting ability to escape.
One idea could be the characters reacting to a modern-day action movie. They might be both amused and critical of the over - the - top stunts and special effects compared to their own gritty reality. For example, Revy could be making sarcastic comments throughout the screening.