Although Koihime event picture is not yet on the server, there is a date on the limited time Halloween Thunder Eidolons:
※Available in Gacha until 20:00 11/21 (PST)
That is probably the patch which will bring Koihime, or Envy if they're skipping it.
EDIT: Oh, yeah, what I meant to say but forgot... is that this is yet another fragment of evidence that we are getting Koihime. Still not confirmed, but they're pushing out Limited Kamihimes so that they won't have two Limited sets up at the same time. As Koihime Kamihime are also Limited.
... at least, that's what I want to believe! Come on, Nutaku! You can do this for us!