Well, during the middle of the series I did give it some thought--IMO, the Tower series starts rather weak, and ends in major disappointment as it doesn't live up to the expectations that King set up for himself.
You have a lot of the elements in the D20M sourcebook. You have the gunslinger advanced class, and you have psionics. You'll probably need to borrow from Star Wars Saga its approach to allowing for force-sensitive characters that aren't from a force-wielding class (because your protagonists aren't necessarily going to be pure psions).
I think it would probably work better to take the basic concepts of the Dark Tower, and then expand it to allow for more fantastic elements. For instance, if I were capable of playing a character that could from any point across time and space, I would probably go for more exotic choices than a bunch of ordinary 20th century humans. And conversely, if I were facing ultimate evil bent on destroying the omniverse, I would probably expect a little bit more in the way of opposition than what the ka-tet faces. Think Dark Tower with a hint of RIFTS.
