Perhaps a fanfiction where Poison Ivy falls ill due to some strange new chemical in Arkham Asylum. Batman, instead of seeing her as just a villain, decides to help her find a cure. This would lead to a journey through the asylum, facing other villains who might want to take advantage of Ivy's weakened state, and also a deeper exploration of Ivy's character and her connection to nature.
One plot could involve Poison Ivy being kidnapped by a new, unknown villain in Arkham. This villain wants to use her powers for their own evil plans. Batman then has to team up with some of Ivy's plant minions (who have a limited form of sentience) to rescue her. During the rescue, we could see a different side of Batman as he tries to understand and communicate with the plant creatures.
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.
They have complex interrelationships. Batman often battles villains like Poison Ivy in Arkham Asylum, and Etrigan might sometimes get involved in these conflicts.
A possible fanfiction could focus on a team - up between Poison Ivy and another Arkham character, like Catwoman. They could form an unlikely alliance to take on a common enemy in Arkham City. The story could show their different motives and how they manage to work together despite their differences. For example, Ivy wants to protect the natural environment of Arkham City which is being destroyed by a new construction project, while Catwoman is more interested in the valuable artifacts hidden in the buildings.
In Arkham Asylum, Poison Ivy's character can be studied through her interactions with the other inmates and the guards. Does she use her charm or her powers to get what she wants? How does she react to the restrictions placed on her? You could also look at her backstory and how her experiences led her to become the plant - loving villain. Maybe she was mistreated in the asylum in the past, and that fuels her current actions. Write about her emotions, her hopes (if any) for a better world for plants, and her despair at the state of the environment outside. This all helps in creating a well - rounded character study.
One idea could be exploring Scarecrow's backstory in more depth. Maybe show his early days in Arkham Asylum, how he was treated by the staff and inmates, and what led him to become the terrifying Scarecrow. Another plot could involve Scarecrow creating a new and more powerful fear toxin that not only affects Batman but also the citizens of Gotham, and Batman has to race against time to find an antidote while dealing with his own fears.
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.
One idea could be inserting yourself as a new inmate with a unique skill set, like being a master of disguise. You could use this to outwit the villains and even Batman at times. Another could be being a doctor who has a secret agenda and is using the chaos in Arkham to further their own ends. Maybe you're trying to steal some of the Joker's toxin for your own evil plan.
Another plot could involve a crossover with a fantasy world. Imagine Batman being transported to a world like Middle - earth. He'd have to use his gadgets and combat skills in a world full of magic and mythical creatures. He might team up with characters like Aragorn or Legolas to fight against an evil force that has some similarities to the criminals in Arkham Asylum, perhaps a dark sorcerer who is as mad and unpredictable as the Joker.
The story shows a complex plot where Poison Ivy uses her powers to control the inmates and Batman has to face numerous challenges to stop her.