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.
Another popular theme is her relationship with Batman. Sometimes it's a romantic relationship, other times it's a begrudging alliance. Writers like to explore how two such different characters interact and what they can learn from each other.
Some writers depict their relationship as a love - hate dynamic. Batman is attracted to Poison Ivy's beauty and her passion for nature, yet he hates her criminal ways. For example, he might be constantly in a struggle between his feelings for her and his duty to stop her crimes.
One common plot twist could be Poison Ivy using her powers in a new and unexpected way to either help or hinder Batman. For example, she might be able to control a new type of plant that has properties that can nullify Batman's gadgets. Another could be a situation where Batman has to make a difficult choice between saving Gotham and saving Poison Ivy from some greater threat. This would put his moral code to the test as he has always seen her as a villain but may have developed some complex feelings in the context of the fanfic.
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.
Sure, she is quite popular. She has a distinct look with her green - toned skin and plant - like features. Her character also has an interesting backstory which adds to her popularity. In the Arkham series, her battles against Batman are always exciting to see.