Batman: Return to Arkham's existence has been leaked already by an excited GameStop employee. Now, it seems that Warner Brothers and Rocksteady are getting ready to officially reveal the game, likely any day now, according to a WB Games Italian Magazine.
Speaking of Italy, it seems they have the edge when it comes to news on Batman: Return to Arkham as GameStop Italy recently posted a listing for the game on their website, alongside a release date of June 6th, 2016. This WB Magazine indicates that the release date is June 10th, 2016, but I'm sure we will know which one is true very soon.
Source: [Hardcore Gamer]