http://www.techradar.com/news/borderlands-3-release-date-trailer-and-news

It’s been 5 years since Gearbox released Borderlands 2 and given that the game was beloved by critics and players alike, we wouldn't be at all surprised if a third proper entry was in the words (and no, the Pre-Sequel doesn't count). 

Borderlands 3 may not have been officially confirmed but at this point pretty much everyone suspects that it’s in the works.