Over on the Arkham City forums, there is a thread with a petition for Batgirl added as DLC with close to 200 signatures. One of the community managers at Rocksteady commented with "We are keeping an eye on this thread!" Subtle hint that Batgirl is next?
Hopefully it comes with something added in the story mode. Would be great to use Batgirl and/or Robin in open world/story gameplay for some more variation.