@ossobuco,
I would lean toward a white Christian male, since that's what all our Presidents were before Obama and being woman would still be viewed, at this date, as being a departure from the norm. How much longer that norm will last is questionable, but it still applies as of now.
One possiblity would be O'Malley, who was unknown until the debates and got almost no support but still gave some good answers, as far as I'm concerned. Plus, he's considerably younger than Hillary, who is a little bit older than optimum.
Going in the other direction agewise, Joe Biden seems to be pretty well liked nationwide, and is definitely a possibility despite his age. Plus, like Rabel said, he crushed WhatHisName in the VP debate last time.