According to Japanese fanblogs, the cover of the next book will have Shuurei, Ryuuki, and an unknown female character on it. The unknown female character is someone of a “highly noble clan,” who will play an important role in the story (holding the key to the novel, apparently!)

Which character is it? I would either guess a) Heki Karin, or b) Yuri.

11/16: Apparently a character who has not appeared for awhile will also appear and do stuff in the next book. This character is also of a noble house and may help Shuurei in some way (is worried about her?).