Apparently at a talk/book signing for his newest,
Under the Dome, King was asked about revisiting the Dark Tower universe and he said he wanted to write one more book that takes place between Wizard and Glass and Wolves of the Calla.
The moderator of the
Stephen King message board confirmed this and elaborated:
Stephen has given me permission to pass along that he has an idea for a new Dark Tower book, the working title of which will be THE WIND THROUGH THE KEYHOLE. He has not yet started this book and anticipates that it will be a minimum of eight months before he is able to begin writing it.