The official @ArkhamCityNews Twitter account indicates that new Arkham City downloadable content is coming December 20th, however, the DLC will feature new challenge maps instead of an elongation of Arkham City's story.
The full text of the twitter post reads, "Brand new DLC coming December 20th with all new Batcave. Jokers carnival and the iceberg lounge also available to download then!"
The Joker's carnival and iceberg lounge aren't new, though. They were actually distributed as exclusive content from various retailers when the game released. Despite this, it's good to see additional content of any kind released for a stellar game such as Arkham City.