AThe Scarecrow, Two-Face, The Penguin, The Riddler and Harley Quinn are the only ones confirmed so far, as well as the titular Arkham Knight. Bane, Poison Ivy, Black Mask, Hush, Calendar Man, the Man Bat and Mad Hatter are also rumored to appear. A Rocksteady spokesperson let it slip that Nightwing will also be an antagonist.
There will most likely be more announced later. Azrael and Quincy Sharp are also rumored. At this point in time (May 2014), there are no other official announcements from Rocksteady.(Feb 2015) A trailer reveals that besides Dent, Penguin, Riddler, Harley and the Arkham Knight, the game will also include Firefly and Poison Ivy.As far as heroes, the only known good guys are Batman, Gordon and Oracle. More may be announced later on. For now, it's all speculation.