It is still an Unnanounced project.
On the Wraparound Jacket of the latest and final Volume of Tegami Bachi: Letter Bee Manga were shown some sketches of some characters.
Well, those characters are from an unnanounced Anime where Hiroyuki Asada (Tegami Bachi: Letter Bee) is working on the design of the characters. The Anime in question will appear on 2016, but is still mistery.
Tegami Bachi: Letter Bee was published on 2006 until 2016.
Via: Anime News Network