Perhaps an OC who is a scientist specializing in the study of the toxins that drive some of the Arkham villains mad. This OC could accidentally create a new strain of the toxin while trying to find a cure. Now they have to team up with Batman to stop the new threat before Gotham is plunged into chaos. Their scientific knowledge would be a key element in the story.
One idea could be creating an OC who is a reformed criminal trying to make amends in Gotham during the events of the Arkham series. They could have a unique skill set, like being an expert hacker, and get caught up in Batman's fight against the rogues gallery while also trying to prove their new - found goodness.
A plot idea could be that Poison Ivy creates a new type of plant - based virus in Arkham that only affects humans who are harming the environment. Batman has to find a way to stop the virus from spreading while also dealing with Ivy's strong beliefs. Another idea is that Ivy discovers a rare plant in Arkham that she believes can save the planet, but it's in a highly guarded and dangerous area. Batman and Ivy might have to work together to retrieve it.
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.
One idea could be a story where Batman and Catwoman team up to take on a new, more powerful villain in Arkham. Maybe this villain has the ability to control the minds of the inmates, creating chaos. They'd have to use their unique skills, Batman's detective work and combat abilities, and Catwoman's agility and thieving expertise to stop the threat.
One idea could be exploring a new villain in Neo - Gotham. Maybe a tech - based criminal who can hack into the Bat - suit's systems, creating a whole new set of challenges for Terry McGinnis. Another could be a story where Terry has to team up with an old - school Batman villain who has somehow resurfaced in the future, like the Riddler, but with a new high - tech twist on his riddles.
Well, you could write a fanfiction where the Joker somehow returns in Arkham Beyond, but he has a new, more terrifying form. Batman must figure out how this is possible and stop him before he causes even more mayhem. Or perhaps a story centered around Harley Quinn trying to redeem herself in the Arkham Beyond setting. She could team up with Batman to take down a common enemy, which would create an interesting dynamic between the two characters.
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 idea could be that the OC is a childhood friend of Bruce's who has a unique skill, like being an expert in ancient martial arts from a far - off land. They could influence Bruce's training and view on justice in a different way. For example, they might introduce him to more spiritual aspects of combat and self - discovery.
One idea could be exploring how the Knightfall event affects Batman's relationships with his allies in the Arkham Knight universe. Maybe some of them start to question his methods more deeply.
One interesting plot idea could be a fanfiction where the Joker somehow comes back to life in Arkham Knight. Maybe he was only in a coma and wakes up with a new, even more deranged plan to take over Gotham. He could start by manipulating the inmates in Arkham again, creating chaos from within the asylum.
In this fanfiction, it could be interesting if Catwoman discovers a secret about the Arkham Knight that Batman has overlooked. She could use this knowledge to her advantage, either to make a deal with the Arkham Knight or to help Batman. For example, she might find out that the Arkham Knight has a weakness related to a certain location in Gotham that was important to his past. Catwoman could then lead Batman there to take down the Arkham Knight.