First of all I think Sansa is going to fulfill Cat's role but she'll do it from a purely revenge based point of view as she won't actually get killed. The entire bolton clan will be slaughtered save Ramsay's baby that, lets face it, Sansa is probably carrying by this point. Which of course would give the revived Stark house direct claim over the Bolton's territories.
PS also Does anyone here think that all the northern houses have even the slightest interest in remaining loyal to the Boltons after the brutal treatment they've received at the hands of the flayed banner? I wouldn't be surprised if Stannis's army balloons with troops still loyal to house Stark as he marches to Winterfell.