I don't think Will Smith will go near westerns ever again... But he would be funny in it. However, he has kids, and likely would object to he's kids seeing him in a movie where's he's called "nigger" repeatedly. Also, the NAACP, Al Sharpton, Jessie Jackson and all the usual suspects would object as well...
I approve of Will Smith in a remake of Blazing Saddles. But I suspect Lily would take one look at him and weep "It wasn't twue! It wasn't twue!"
Would work. But I have a horrible suspicion they'd cast Ben Stiller in Gene Wilder's role, and that would make baby Cthulhu cry...