According to Gearbox CEO Randy Pitchford, "It's no secret, obviously there's going to be another Borderlands". Pitchford made the statement at this weekend's PAX East and further revealed that Battleborn art director Scott Kester will also be working on the project. Mikey Neumann will be the author of the storyline.
While the statements are encouraging, there is no further information about when it might possibly be released, though development is on tap to begin after Battleborn is released and its DLCs are complete.
You can read more at Eurogamer.