Intrepid Atlus followers have two main guesses: First, a new Growlanser game, part of the tactical RPG series which Atlus has published in Japan in the past. Or (and this seems somewhat more likely) a game called Gungnir (subtitled Inferno of the Demon Lance and the War of Heroes), which Atlus published earlier this year in Japan. Most of the titles in the Dept. Heaven series have already been published in the US in some form, so it makes sense that Atlus would be publishing this one as well.
Did we guess right, Atlus? Enough with the teasing!