Idris Elba would be a pretty cool choice, I've finished the first 3 books in the series and am currently reading the next one, I would really like seeing him play Roland Deschain. After seeing Deadpool break records at the box office as an R-Rated film i would live to see an R-Rated Fantasy film, since it's such an uncommon genre for films and often aren't very well received, with a few exceptions like the fellowship trilogy. What I'd would be really worried about is what kind of budget it would get as a film series because decent effects can either make or break these kind of films.