The story has been revolving around the word "what I want to do". That's Honoka's motivation when she formed the group, and what led the others, notably Hanayo, Maki, and Eli to join as well.
Now the question's being directed to Kotori. We all know that she really wants to be with Honoka and friends. While the possibility of her totally leaving is open, things indicate that she'll stay for Muse itself. Not to mention she's the costume designer and also dance steps creator within the group (although someone else could take that role).
It's difficult to make her stay without cheapening the previous drama (but hopefully Sunrise can pull it well), but if she leaves, there are too many things left open too. That's why I don't like why it has come to this kind of development.
I personally want her to stay. Oh, the wait is killing me
