I'm sure most people know about it. But here's what I've seen from articles.
Joker WILL be in it played by Bioshock Infinite voice actor, Troy Baker. Anarky will be in it and will play a role similar to the Riddler's in the other game. Bane is comfirmed but probably won't be AS ripped since they're going for a "More like the comics" look for him.
If you haven't seen the trailer, it looks like Black Mask, Deathstroke and Deadshot will be prominent antagonists. Deathstroke is also confirmed to be playable at some point. Don't know if it'll be like Nightwing or Catwoman were in Arkham City.
"Taking place before the rise of Gotham City's most dangerous villains and assassins, the game showcases a young, raw, unrefined Batman as he faces a defining moment in his early career as a crime fighter that sets his path to becoming the Dark Knight. As the story unfolds, witness identities being formed and key relationships being forged."