Now that this project has fallen apart, it's got me thinking about a different approach.
What if the filmmakers approached this the way Marvel is doing the Avengers movies? In that case you have several separate stories which combine from time to time. There will be an Iron Man 3, a Thor 2, and an Avengers 2 and 3 in all likelihood. We haven't seen this in movies before to my recollection.
So imagine one filmmaker making Dark Tower 1, another filmmaker making 'salems Lot, another making DT 2, another making The Stand, and so on, then to have the different stories and characters mingle down the line?? This is purely fantasy on my part, but still, how cool would it be to see Father Callahan show up in a later Dark Tower movie? Or to go to Insomnia just to get a glimpse at a future DT nemesis? Would be cool, just sayin'.