Achievement Trophies,discovered by PS3Trophies.org, indicate the Harley Quinn themed DLC will be a mini-campaign, [i.e. an actual story] rather than another DLC focused on beating up bad guys in inventive ways. Of course, there's no indication as to "what" story this will tell, as this could be a continuation of the "bun in the oven" Easter Egg from the main story or could be something else entirely.
In previous DLC's named after a character, [see Catwoman, Nightwing, Robin, etc]. that particular character was playable so you may be able to play as Harley. One thing that's not speculation is that you will be able to play as Robin. One of the Trophies you can achieve requires using "5 different Quickfire gadgets in one fight as Robin," so you'll definitely be able to play as Tim Drake.
What's most encouraging here is that this could be the tipping point for more story DLC with other characters as was promised by Rocksteady during the games initial launch.
Batman: Arkham City is a 2011 action-adventure video game developed by Rocksteady Studios. It is the sequel to the 2009 award winning video game Batman: Arkham Asylum, based on the DC Comics superhero Batman. The game was released by Warner Bros. Interactive Entertainment for the PlayStation 3, Xbox 360 and Microsoft Windows. The game was officially announced during the 2009 Spike Video Game Awards and was released worldwide for consoles, beginning in North America on October 18, 2011, with the Microsoft Windows version following on November 22, 2011. A Wii U version is set to be released in 2012. A spin-off iOS game, Batman: Arkham City Lockdown, was developed by NetherRealm Studios and released on December 7, 2011.
Arkham City was scripted by veteran comic book author Paul Dini, who also scripted Arkham Asylum. It features a main plot and side-missions that develop their own sub-plots. The main storyline revolves around Batman's imprisonment in Arkham City, a sprawling new super-prison five times bigger than the original asylum and enclosing a wide variety of industrial districts, iconic locations and Gotham City landmarks. He must now stop mishaps occurring around this chaotic enclosure caused by Gotham's most notorious super criminals. The game has received universal critical acclaim and is one of the highest-rated video games of 2011 according to review aggregator Metacritic.