Earlier today, the Batman Arkham Facebook page posted a "Save the Date" image informing fans that a "special announcement" will be coming on December 31st. While the announcement could be anything, there are two likely candidates: the upcoming Story DLC for Batman: Arkham Origins (part of the Season Pass) or the "next-gen" Arkham game that Rocksteady is currently developing. It seems like an odd time to first unveil a new game, so my bet is on the Story DLC. Both have been shrouded in secrecy up until this point.
If you've completed the story campaign of Batman: Arkham Origins, you'll know that there is a post-credits scene that hints at a storyline for future DLC, which seems to have made most fanboys very giddy. If you don't own the game, don't worry, I won't spoil it, but be sure to check out my (equally spoiler-free) review. I personally loved the game!