Did you buy the season pass to Batman Arkham Asylum? The £33 pass gives you access to 6months of content.
If you have then you'll get to play as Batgirl early. July 17th is the date for you to get Batgirl: A matter of family. This is the first time players are able to play as the sidekick of the man himself.
If you didn't by the season pass then you'll have to wait til the 27th July to grab the first piece of content for the last oorah of the series.