The game isn't out yet but there has been leaks about what the DLC will feature. Recently Amazon Germany showed that if you bought the game via them that you would get the "Blackest Night" based on the DC Comics run where Nekron brings back to life dead superheroes to destroy all life from the universe, because he's a bit of a dick. Believe it or not there were quite a lot of characters that stayed dead, just not many that were well known.
The pack also comes with the game if you pre-order from Bestbuy.
Other packs revealed so far are Superman: Red Son pack (based on the run where Superman is Russian...it's good honest!) which includes various alternative skins from that comic run as well as 20 extra missions. You get that by pre-ordering it from Game, Gamestop & EB. Pre-order from Walmart and you get a Arkham City Skin Pack
Sourcey
The pack also comes with the game if you pre-order from Bestbuy.
Other packs revealed so far are Superman: Red Son pack (based on the run where Superman is Russian...it's good honest!) which includes various alternative skins from that comic run as well as 20 extra missions. You get that by pre-ordering it from Game, Gamestop & EB. Pre-order from Walmart and you get a Arkham City Skin Pack
Sourcey