SEGA Japan has just launched a new teaser site for an upcoming game that will be revealed on September 16th. Just in time for the Tokyo Game Show! The URL for the teaser site is http://code1935.jp.
SEGA Japan recently registered valkyria3.jp as a domain name not too long ago. Also, 1935 is the year of the original Valkyria Chronicles game. I think it is pretty safe to assume this game is going to be Valkyria Chronicles 3!