Suspicion has been lurking for some time that King was not quite finished with the Dark Tower series. Speculation died down when it turned out the upcoming "Ka is a Wheel" was a game..
Lilja's Library quotes Zach Dionne thusly: "King stated he will write another Dark Tower novel. It will center around supporting characters and revolve around some important events between Wizard and Glass and Wolves of the Calla.".
That's not much of a hint. But, it's enough to speculate! Firstly, "important events between Wizarda and Glass and Wolves of the Calla":.
Here's what we know happened to the Ka-tet between Wizard and Glass and Wolves of the Calla:.
1. Only six weeks elapsed between the books. (or seven weeks. Bev Vincent's books says seven. My wife says six. In my world, my wife is always right. Because she cooks dinner.).
2. After Roland tells the story of his younger years and his first love, they are back on the path of the Beam..
3. We know that the ka-tet lost some time in Wizard and Glass..
"The sky was clear when we went inside the palace, he thought and realized time had slipped again. How much or how little he wasn't sure he wanted to know -- Roland's world was like a transmission with its gear teeth all but stripped away; you never knew when time was going to pop into neutral or raceyou away in overdrive. Was this Roland's world, though, and if it was how had they gotten back to it?" (p.771-772 W.G. hardcover).
4. They also walked in their sleep..
5. Someone gave Jake food -- a chicken leg..
6. They continue on, walking along the path of the beam. They are disoriented.
So what's new and big that happened between the books? Only the King knows.
I suspect the new book is very much in the world of the Dark Tower, but does not use Roland or Eddie at all. You can check Liljas to see what character is promised to be there. Remember the Zach Dionne quote, "It will center around supporting characters and revolve around some important events between Wizard and Glass and Wolves of the Calla."
The Dark Tower involves characters beyond the seven novels. This includes Insomnia and Little Sisters Eluria. Point is: Anyone can pop up!
Of course, Wizard and Glass was more focused on "supporting" characters. It was a background novel as much as anything. It did not advance the immediate story, but deepened the characters. So it's interesting that another novel with supporting characters would come directly after Wizard and Glass.
Okay, so there's really nothing new here. I always feel overwhelmed with the Dark Tower. I can't keep up. I feel like a D- student in a room full of A+ graduates.