According to Warner Bros. official Facebook page, an announcement related to Batman Arkham will be made on 31st of December. Along with this Warner Bros. also posted a new teaser image which you can view below.
As this announcement was made on Batman Arkham Facebook page, we can expect an announcement to be related to Batman Arkham City.
The last day of this year would be ideal for Warner Bros. to make an announcement for the next Batman: Arkham City title. Let us wait for December 31st and see what Warner Bros. is upto.