The Arkham series loves making its villains playable as DLC. In Arkham Asylum, you could play as the Joker in free PS3-exclusive DLC. In Arkham City, you could play as Catwoman in a story integrated into the main campaign--provided you bought a copy of the game new. And in Arkham Origins, you'll be able to play as Deathstroke, provided you pre-order the game from Amazon.
An ad on Amazon discovered by IGN talks up the unannounced "playable Deathstroke DLC." Pre-orders from the online retailer will entitle you to the add-on content, as well as $10 of credit.
It's unclear if the promo will be exclusive to Amazon, or if other retailers will be able to get in on the pre-order DLC. Either way, it's unlikely to be locked away for too long. For example, when Best Buy got exclusive access to Robin DLC for Arkham City, it only took a few weeks before the general public could buy it.