While it hasn't been officially announced, some Harley Quinn DLC trophies have popped up on PS3Trophies.org.
The already-released Robin and Nightwing DLC packs for the game have simply been challenge maps with new playable characters and no new story missions. However, that may not be the case this time.
Trophy criteria includes things like "Find a way into the secret base" and "Clean up the Dry Docks" which suggests that this will be a new campaign. The trophy description "Use 5 different Quickfire gadgets in one fight as Robin in Harley Quinn's Revenge" also seems to imply that Harley Quinn herself won't be playable, but Robin will. Also that the DLC will be called "Harley Quinn's Revenge."
Of course, this is all an educated guess. One thing we can probably say for sure is that the DLC is coming. Everything else is speculation.
