It seems Rockysteady Studios isn't quite finished with Batman: Arkham City. A leaked set of Trophies suggests that they'll be releasing a new downloadable content pack centered around Joker's main squeeze Harley Quinn.
Lost Property - No crimefighter should be without this
Breaking and Entering - Find a way into the secret base
How's It Hanging? - Clean up the Dry Docks
The Last Laugh - The joke's on who?
Frequent Flyer - Zip Kick 3 different thugs
Battering Ram - Shield Bash 5 different thugs
Snap To It - Snap Flash an unarmed thug, an armed thug, an environmental object and a Titan
Bomb Squad - Defuse all bombs in 3 minutes or less
A Few New Tricks - Use 5 different Quickfire gadgets in one fight as Robin in Harley Quinn's Revenge (1)
Party's Over - Destroy all Harley Balloons
The descriptions of some of the Trophies suggest that the DLC is story-driven. It won't just be, say, a series of challenge rooms. Several months have passed since the last Arkham DLC was released so I wouldn't be surprised if Rocksteady had been cooking up something big during that stretch.
One of the Trophies also mentions that you'll take control of Robin at one point. The Boy Wonder was previously featured in his own challenge room DLC pack. Considering how bad-ass his Arkham City incarnation is, though, I won't complain.
Now that the cat's out of the bag, I'd expect Rocksteady or WB to announce this DLC soon.