Well I guess you can count me in for either a time-skip, demon threat, or a combination of those.
Imagine a time-skipped Nanami. Then she uses a hero/demon crystal. She'll transform into Nana Mizuki herself! Milhi might even get the chance to do a
duet!
IT WILL BE THE GREATEST CONCERT FLONYARD WILL EVER HAVE IN ITS ENTIRE EXISTENCE!!!
Other than that...I wonder what it will officially be called.
EDIT: Oh I also hope that we get a chance to see some of our beloved cast turn to actual furballs... and hopefully nothing strange like that transformation scene.