On the Booklist I have been recently talking about the Dark Tower books, since I have just finished WaG. We also discussed theories of Stephen King book cosmology at work one night, so I think I'm going to propose some of my own theory on what will happen to the ka-tet in the later installments of the books, and how it ties into King's writings as a whole.
Read or don't.
First, I think that it doesn't really matter whether or not Roland's world is the future of one of the other books of King's, only that it intersects these as it sits at the top of all his realities, or at least at the center of the spokes.
In the Dungeons and Dragons game of Planescape there is a city that sits at the center of all realities called Sigil, and it is as the center of a wheel in all reality. There are doors from Sigil that go to everywhere, and I think that this is what Roland's world is.
One of the possibilites that I see is that the Dark Tower is merely reality itself, and that King himself is the Crimson King and when Roland and his Ka-tet reach the top, or the center this is what and who they will find. Elsewhere on the net, I read someone's idea that the reason that the worlds are moving on and falling apart is that King is getting older and they are fading as he gets older.
Could be true.
One thing that I do think will play into the series are the Dark and Light hotels from the Talisman, and I think that the Talisman itself may have been part of Maerlyn's Rainbow. I don't know if King consciously thought this when he (or Straub) wrote that part of the Talisman, but that is how it may play now.
In the prologue to the Wolves of the Calla, he tells of children being taken away by wolves on iron horses. I believe that these are wolves working in the company of Flagg and under the Crimson King, as we saw in Black House, where part of the action took place in an area that I think is reminiscent of the land known as Thunderclap, or perhaps is Thunderclap. The roont children that come back have been touched by otherworldly power, much like Collie in Desperation and other characters that were worn by Tak.
I also think that Tak will play a role in the final drama and probably has something to do with the Crimson King himself. I also think that it is likely that John Farson, Marten and Flagg are all the same man, as we know that Flagg and Marten are.
It is also possible that Sunlight Gardner is or was one of the lieutenants of the Good Man, or perhaps he was just one of Flagg's underlings in the Territories, much as Trashcan and Roy were in the Stand.
What will happen when they reach the tower? I don't know.
I do think that almost every book that King has written ties into this series somehow though, if only through a character having the touch, also known I think as the shine.
Here are some of the ways that I see some of the books as linking.
Desperation and the Regulators--Tak and the idea of the power inhabiting a body and blowing it up They take children in the Wolves of the Calla, because they won't use up as quickly as adults, perhaps even the great Lord Perth was a human touched by one of these spirits.
IT-It concerns an otherworldy entity, envisioned as a spider at sometimes, who could be one of the guardians on the wheel, I don't think that King has listed all twelve animals. The turtle is most certainly tied in, and the symbolism of the ka and ka-tet are too. Perhaps It is another manifestation of Flagg?
From a Buick 8-The world that the car opens onto is strangely reminiscent of the Waste-Lands that Blaine drives over.
The Shining-The shine itself, known as the touch in the Dark Tower Books, and the overlook hotel, which could be a mirror of the dark hotel in the Talisman.
Black House-There are many direct ties there, from the Little Sisters, to Parkus's appearance as a pseudo-Gunslinger, and even discussion of Roland and his quest.
The Stand--Again direct ties, as Flagg appears, and the ka-tet walks through part of the Stand's Kansas.
Insomnia--There is much discussion of the Tower and what it means in this book, thought I need to read it again, it's been awhile.
Needful Things--Is Gaunt Flagg, or is he perhaps just another Demon?
Eyes of the Dragon--Flagg, and the fact that Roland mentions meeting characters from this book mean that they overlap directly, though the world of Delain is not neccessarily that of Gilead.
I think that's enough for now.