Square Enix has opened a mysterious teaser website for a new secret title that they will announce in 153 hours from now (Friday 30 March).
On the teaser website you can see a trailer with artwork of the new game and Masaki Hirooka and Yusuke Naora are credited as the illustrators. Also some keywords (like crystal and summons) are showed.
Edit: Seems to be hours and not days.
Update: The game is called Crystal Conquest and will be a real-time battle strategy browser game for Yahoo! Games in Japan.