After Byron Howard's Zootopia was confirmed for the March 04, 2016 release date, it wasn't known which film would occupy the other 2016 slot. Now according to the member Mr. Azevedo on the Brazilian message board AnimationS who claims to have spoken with Nathan Greno, the film planned for the fall of 2016 is his. However, since we can't verify the accuracy of this claim, we'll only be treating this as a rumor for the time being.
This member also shared a photo that Nathan Greno posted on his Facebook account on May 17, 2013. Nathan had the following to say about the project.
Source: https://www.facebook.com/photo.php?fbid ... .621242100Nathan Greno wrote:Trust me, we are hard at work on the new film! I wish I could say more -- I'm REALLY excited to tell everyone what it is! If you enjoyed Tangled, I believe you'll love the new project.