Girls' Generation's Yuri revealed in a Youtube live that she was supposed to come back with a special album, but the release is ultimately delayed.
In the clip below, (at 20:15 on the Youtube live), she describes her comeback as a special album, with another member of Girls' Generation, and said that they had a song picked out along with a concept for the album but for various reasons, It is delayed. She then reassures Sones and Mandoos (Yuri's Fanclub Name) that it will be released sometime soon.
Honestly I can’t even guess who the other member is, there are just too many great pairings... I mean SHY would be my dream come true.