Are you ready for this? Your wallet better be, because a Batman: Arkham City Harley Quinn DLC is on its way.
Word of the DLC was leaked out with the release of a trophy sheet, with the title possibly being "Harley Quinn's Revenge" based on one of the trophies. Judging from the attacks and the feats you're required to perform, you'll be playing the role of Robin in the new single player DLC.
Unlike the previous DLCs, which were just arenas and additions to the game's challenge mode, this one appears to be a full fledged single player add-on.
Hopefully, it won't be the last DLC they make for Batman: Arkham City—because I'd very much like to play Nightwing in the single player campaign.