In a recent interview with The Guardian newspaper, Sefton Hill (Game Director at Rocksteady) hinted at story based DLC for Batman Arkham City.

Any story-based DLC?

[Smiles] I don’t think I’m allowed to talk about that at this stage.

The prospect of story based DLC is very welcome in the game. Robin and Nightwing have already been made into playable characters although only for the challenge map modes in the game. It is quite likely they will be given their own story arc in Arkham City. You may also encounter new villans from the Batman universe and fans have reported that there are many buildings with accessible looking doorways which are infact inaccessible that could be locations for new DLC.

The trophy available for Calendar Man to visit him on specific dates during the year adds to the fact that the game will be supported well via DLC and keep you playing for a long time.