An image tweeted by Gearbox lead writer Anthony Burch might have inadvertently leaked the next story add-on for Borderlands 2. As you can see in the image below, a poster in the background shows Tiny Tina's Assault on Dragon Keep.
This seems to coincide with Gearbox's PAX East teaser which involved Tina playing a game of "Bunkers & Badasses," the Borderlands version of Dungeons & Dragons, with original Vault Hunters Mordecai, Lillith, and Brick.
The tweet has since been deleted (not before a screen grab), which suggests Tiny Tina's Assault on Dragon Keep could be real. When Gearbox originally revealed the Borderlands 2 Season Pass, they said the fourth story expansion would release by June, meaning we should get official word sometime soon. If Dragon Keep is real, this would be the fourth and final piece of Season Pass content.
