The Official Arkham City Twitter Account posted this update:
After you click on the link you are taken to this image:
With the Harley Quinn DLC now out, which you can check out the review for it here, and the success on Arkham City still riding high, it would not be unheard of for another DLC to come out.
In Arkham City there was an Easter Egg that alluded that the Scarecrow survived his attack by Killer Croc from Arkham Asylum. Could Rocksteady be telling us that he will have his own DLC next? It certainly seems this way, and will be much anticipated.