Masked. This is a game, not another case of political correctness gone mad, there's no NEED for a woman, TF2 is MEANT to be a joke, in my opinion - whatever valve can make funnier in the long run - it's all good with me. The masked style has always been fun - the mystery is amusing, why ruin that?! It's one of the main fads that TF2 drag along behind it.
Knowing valve they could make ANYTHING funny, but it sure doesn't mean they have to ruin things in the long term for a quick meet the Pyro amusement where the mask finally comes off and reveals -insert pyros identity here-.
I do imagine valve could tease us with the masked bit - perhaps having the mask come off slowly and dramatically - then followed by a smug spy/blue team laugh as they're mucking about with the pyros costume and mask. Then the real pyro comes in and the real meet the pyro begins!