User blog comment:CureLightning/Part 5 announced??/@comment-26426093-20161229013035/@comment-3487190-20161230105841

If going with "type theory", it's not like Yuzu+Ako or Ako+Ruka can happen in the same time. But then there has to be a new character to fill the space, which is the issue now.

Koharu isn't the main problem, anyway, as she's not returning in short time.