According to Goodreads:
Arcana Chronicles, Book 3.5
by Kresley Cole
Coming Summer 2016.
"In DAY ZERO, we'll meet new Arcana who'll be very influential in the upcoming books, and we'll see how some of the players we already love/hate survived the Flash."
I have not been able to find any other info online about this- yet, but it looks amazing and super legit. I really need to not be such a wimp and just finish Dead of Winter!