The Harley Quinn's Revenge DLC coming to Batman: Arkham City has been detailed, and it sounds like a departure from the game. Instead of an open-world affair, the DLC is story driven, initially placing you in Robin's boots and tasking you with finding Batman, who's been lost chasing Harley Quinn after the final events of the main game. The story time-shifts, explaining where Batman is and what he's been up to by switching viewpoints a few times.
Robin still doesn't have the chance to wander Arkham in full - he'll only be able to explore this content and any previously released challenge maps. Harley Quinn is scheduled to get her revenge on May 29, for $9.99 on PSN, or 800 Microsoft Points on Xbox Live.