One interesting storyline could be about Batman having to face a new villain that was created by the combination of the fear toxins from Scarecrow and the technology of Hugo Strange. This new villain has the ability to manipulate people's minds on a large scale and Batman has to use all his detective skills and combat abilities to stop it.
One possible storyline could be that in the dark and gritty world of Arkham Origins, the Joker discovers a stash of strange, magical candies. These candies have the power to make people see their worst fears as if they were real. He then uses them to torment Batman and the other inmates of Arkham. Batman has to not only deal with the chaos in the asylum but also figure out how to stop the effects of these candies.
You could also do a 'what if' scenario. For instance, what if Batman had to team up with some of the Arkham inmates, like Harley Quinn and Poison Ivy, to stop a greater threat that was endangering not only Gotham but also the inmates' own lives? This would involve exploring their complex personalities and how they might interact with Batman in a different context.
One storyline could be that Jason discovers a plot against Batman early on. He then sets up a counter - plan and at the crucial moment, jumps in to save Batman from a deadly trap. This would show Jason's intelligence and his lingering care for Batman despite their past issues.
One interesting story could be about Batman's internal struggle with the darker side of Gotham while facing the Arkham Knight. It might explore how he overcomes his doubts and fears in a more personal way than in the game.
Some 'batman arkham city fanfiction' might center around the creation of new villains or the backstories of the existing ones. For instance, a story could be about how a certain minor character in Arkham City became a villain. It could delve into their motives, their past traumas, and how Arkham City's environment influenced their transformation.
One interesting storyline could be a reimagining of Batman's first encounter with the Joker in the 1939 fanfiction context. Maybe the Joker starts as a small - time criminal and Batman has to figure out his true motives early on.
One of the main storylines is Batman's fight against the various supervillains who have taken over Arkham City. He has to deal with the likes of the Joker, who is always causing chaos. Another storyline involves the mystery and secrets hidden within Arkham City, like the dark pasts of some of the inmates and the experiments that might be going on there. Batman also has to face moral dilemmas as he tries to protect the innocent and bring justice in this lawless place.
There are also storylines that explore Batman's detective skills. He has to solve complex mysteries in Arkham, like figuring out the connection between different criminal activities that seem to be part of a bigger, more sinister plot. This often leads him to uncover secrets about the inmates and the history of Arkham itself.
In some fanfictions, the plotline might be that there's a hidden treasure in Arkham City that both Batman and Catwoman are after for different reasons. They start as rivals but as they face the various threats in Arkham City, like the other inmates and the corrupt guards, they realize they need to work together. This story could explore how they negotiate their differences and build a stronger bond in the context of Arkham City.
One idea could be exploring the backstories of the lesser - known characters in the 'Batman Assault on Arkham' universe more deeply. For example, you could focus on Killer Croc's life before Arkham and what led him to become the way he is. Another plot could involve a team - up between unexpected characters from the movie, like Harley Quinn and Deadshot, against a new threat in Arkham that the Bat - family is unaware of.