The original draft of the script had the role of Poison Ivy (played by Karina Smith), but the writers decided to rewrite the script without the character's appearance.
Early in production, the writers and directors ambitiously planned to make a sequel. The plot line was planned to be Joker and Riddler each breaking themselves and other patients at Arkham out and competing to be the top ranked criminal. Joker's team would have consisted of himself, Harley Quinn, Scarecrow and Killer Croc. Riddler's would have consisted of himself, Two-Face, Poison Ivy, and Firefly, who would play a key role to the story as the person who burned down the Joker's girlfriend and child's house.
A third film was in talks during early production as well. The plot line would have been Penguin (the top criminal) joining the crime war with Bane, Catwoman, and Mr. Freeze on his side. There were also talks of bringing in Dick Grayson, a.k.a. Robin. Neither the second film nor its sequel are going to be made.
When Bruce Wayne is reviewing the security footage of Joker breaking into Wayne Enterprises, two tickets to "The Flying Graysons and Company Circus" can be seen on his desk as a cameo for Dick Grayson, the first Robin.