See that image above? Well, it was posted on Batman Arkham's official Facebook page along with instructions to "visit this page on December 31st for a special announcement."
Could it be some Arkham Origin's DLC? A new entry into the series itself, perhaps?
Batman Arkham [Facebook]
Here's a half hour of the Arkham Origins Mr. Freeze DLC, if you want to watch that
11:45 AM on 04.15.2014