I don't think he is a wild card, because his purpose is evil. The wild card of insomnia was Ed Deepnau, because he didn't have any. Walter has one; doom. Those who are evil are doomed in Stephen King's novels. Except for Needful Things. I think as long as we have money and desire to own what is rightfully ours, there will always be a Mr. Gaunt somewhere out there...
I actually wonder if Mr. Gaunt was Flagg...