Ruby. She has a very open - minded and empathetic nature. She would likely be the first to try and reach out to the Arkham Knight, believing that there is good in everyone. She might see his skills and think that, if redirected, he could be a great asset in fighting the Grimm.
Another plot could involve Weiss using her family's resources to create technology similar to what's in Arkham Knight's world to combat a new type of Grimm. And the Arkham Knight himself might be interested in this tech and try to steal it, leading to an interesting conflict and alliance between Weiss and the other RWBY characters against him.
Well, the Arkham Knight Bane is a complex character. When paired with an OC in fanfiction, it gives writers a chance to explore different aspects of his personality. Maybe the OC is someone who tries to reform Bane, or perhaps they have a shared past that is only revealed through the fanfic. The fanfiction can also play with the power dynamics between them. If the OC is a weaker character, how does Bane react? Does he show mercy or is he as ruthless as in the game?
There's a fanfic called 'Arkham Knight: Riddler's New Game'. This one is really interesting as it puts the Riddler in a situation where he has to team up with Batman, albeit unwillingly, to stop a greater threat in Arkham City. The dynamic between the two is explored in a whole new way, with the Riddler's riddles becoming more of a tool for cooperation rather than just a means of taunting Batman.
Well, there's a fanfiction called 'Jason Todd: Beyond the Arkham Knight'. In this story, Jason tries to find his own identity separate from the labels of Arkham Knight and Robin. It has some really cool action scenes and also explores his friendships and enmities in Gotham. And then there's 'Arkham Knight's New Path' which is about Jason forging a new, more positive path for himself in Gotham.
There are fanfictions that explore Poison Ivy's backstory in the Arkham Knight world more deeply. For example, a story might focus on her early days and how she came to have such a strong connection with plants. This can add a lot of depth to her character in the context of the game's lore.