potential
i'll admit, i'm not acquainted with the Sin City line, but enjoy Frank Miller's work in batman. it would be great to see you add plot to this piece, or redo it altogether with a completely different story. i'll tell you one thing, if you do "the dark night returns" you'll make it onto my favorite authors list instantly
bonus points for: copying the comic (er, graphic novel) style perfectly and making it as gritty as Frank Miller would have wanted it.