Often, the development begins with a mutual fascination. Batman is intrigued by Poison Ivy's powers and her unwavering beliefs, while Ivy is drawn to Batman's strength and mystery. As the story progresses in fanfiction, they might have shared adventures. For instance, they could be forced to work together to stop a villain who is harming both the city and nature. During these adventures, their feelings grow as they rely on each other and see the good in one another. It's a slow - burn process where they gradually let down their guards and fall in love.
Well, in a lot of fanfiction, the development of their romance is complex. At first, they are at odds with each other. Batman sees Ivy as a criminal due to her illegal activities in the name of nature. But then, things start to change. Ivy might show Batman the beauty and importance of nature in a new light, and Batman might show Ivy that there are other ways to achieve her goals. Their relationship evolves as they start to communicate more, have deep conversations about their values, and eventually, their initial animosity turns into love.
In some fanfiction, it starts with a chance encounter. Maybe Batman is investigating a crime related to Ivy's plants, and he sees a different side of her. This initial meeting plants the seed (no pun intended) for their relationship. Then, as they interact more, they start to understand each other's motives better. For example, Batman realizes that Ivy's actions, although extreme at times, are rooted in her love for nature.