Is Idris too good, though? I always felt that The Gunslinger was a good coming-out party for Roland. A silly notion, since it's his first novel, but given the information passed along in later novels, it all kind of makes sense. He's a badass, but he's not quite the badass he is in the subsequent books. Would Idris' gravitas ruin that vibe? He's like Denzel Washington* or Tom Cruise# (both doing it in very different ways) in that you immediately get their 'best' and they don't play downtrodden or mild characters very well.
* Not to compare Idris only to another Black Actor, but Denzel was my best example of someone not able to play down his awesomeness. Even in The Equalizer, and Taking of Pelham, everyone in the room should have known he wasn't just Joe Smith Everyman. It's like if you walked your 5 year old into his or her kindergarten classroom and 'Ahnold' greeted you with a firm handshake. You know sh!ts about to go down.
# I know Tom Cruise is very divisive and you either like him or hate him, but at least you know what you're getting with all of his movies. Same with Will Smith and/or Mark Wahlberg. You're getting exactly what you expect (and, in some sense, hope for) when they are your headliners.