In regards to story details, NintendoEverything notes that it takes place on Christmas Eve, with mob boss Black Mask putting a bounty on Batman for unknown reasons. With the top assassins from the DC Comics Universe on the hunt for Batman, the mob also pays off the cops to hunt down Batman as well. Also, Rocksteady Studios (who created the original Arkham games) are working on a "tightly wrapped" project. In regards to Arkham: Origins Blackgate, the game will take place right after Origins, and take place only in Blackgate Prison. They also note that the free-flow combat system will be extremely similar to the console ports. Check out tons of details on Arkham Origins by clicking the source link.
Arkham Origins
Arkham Origins: Blackgate -- 3DS/VITA