This is just pure speculation. It is not based on anything in the novels or other Dresden Files shorter fiction. Having a short story or novella that takes place after Twelve Months could allow Jim to do something Jim claims he hasn't done in a Dresden Files novel, create a cliff hanger that leads directly into the next novel Mirror Mirror.
I'm thinking of the late 80's, early 90's TV show Quantum Leap, in which each episode would end with the main character finishing whatever mission he was on and then leaping into someone else's life and body, often in the middle of a dangerous situation. Dr. Sam Becket would always respond to these surprises by saying "Oh Boy" with varying levels of surprise and stress in his voice.
The next story or novella may be titled Out Law. It seems a reasonable possibility that a story with this title could involve the White Council. The last time Harry spoke to anyone in the Council it was Carlos Ramirez, who appeared to blame Harry for the deaths of Wardens Wild Bill, Yoshimo and the disappearance of Warden Chandler. With those facts in mind, I could see Harry being reminded of this, along with whatever else he might be accused of in the story itself, though eventually managing to exonerate himself. The Quantum Leap twist would be Harry being summoned into an incredibly dangerous situation where someone; more likely a bunch of someone's, are trying to kill him, but Warden Chandler is also there and the only person who can save Harry. Que Harry saying "Oh Boy."
OK, maybe Jim isn't a Quantum Leap fan, but I could still envision Out Law being used as a setup to the beginning of Mirror Mirror. With luck, by the time Out Law is released Jim will have finished be close to finishing the next Cinder Spires novel and be ready to start on Mirror Mirror. You may have noticed Jim hasn't done many personal appearances this year. Other than Dragon Con in Atlanta during the upcoming Labor Day weekend, I don't believe he has done a major convention this year. I know he cancelled an appearance this April in Chicago. With luck, maybe that means Jim is making great progress with that third Cinder Spires book. I know I'm looking forward to that novel because I very much enjoyed the second one, but also because the sooner Jim finishes it, the sooner he can start on Mirror Mirror.