This game. This game is a very good example of a game announced way too soon.
Why am I using this game? Well.
First, it was announced for release in "Summer 2013," which was later revealed to be "July 11, 2013." Then the game was delayed into "Autumn 2013" due to "quality issues." Then the game was delayed to an even vaguer "2014" release. After that delay, we heard nothing about the game. The site, as well as everything else relating to the game went without an update until TGS 2014, when ATLUS finally showed a new trailer for the game at the SEGA booth, which finalized the release date as "January 29, 2015." Many people assumed the game was canceled for various reasons, some being that the game wouldn't sell well due to the anime being poorly received. (The anime adaption of Devil Survivor 2 was poorly received by critics and fans alike)
That's right. From October 2013 until September 2014, we heard nothing about the game. It had huge amounts of info in Famitsu, Dengeki, and other gaming news sites in Japan and just disappeared after its delay from July 11th, 2013 until TGS 2014.
There are theories that Index Corporation (the former parent company of ATLUS) had something to do with the delay, but we'll never know for certain. And yes, the game is now out in Japan, as it's January 29th, 2015 over there.