Batman: Arkham City released last year, and received a few downloadable content packs featuring Robin and Nightwing as playable Challenge Maps. That would seem to indicate the end of that particular DLC, but a new set of Trophies seems to indicate we'll get a new story episode. Ending spoilers follow.
The new PlayStation 3 Trophies found on PS3Trophies.org (via CVG) point toward featuring the Joker's groupie, Harley Quinn. The most significant clue likely comes from a Trophy called "A Few New Tricks," which reads: "Use 5 different Quickfire gadgets in one fight as Robin in Harley Quinn's Revenge." This could point toward post-game content, since the game's ending certainly leaves Harley plenty of reason to seek revenge.
We heard some indication of continuing plot episodes before the game released, when Batman voice actor Kevin Conroy said he was recording extra voice work for downloadable episodes. Those episodes are taking their time coming to fruition, but here's hoping they hit the market. Serial pieces of plot should fit a comic property to a T.