As much as I like McGonagall. And she is my Favroite Teacher at HogWarts I think that She Is too worried about the school to spend time leadind a secret order.
Besides where is the love for Moody after the Sirius death he did look like he kind of took the chains. When it came to the Durselys any way.
But the sad truth there is no one that comanded the respect that DD did. So mabe that means that Harry will step up and take charge.