I think it was in the Sam Mendes-curated Empire issue in which it was stated that Fiennes vehemently disagreed with the idea that M could be part of the Spectre-scheme that Denbigh finally fronted.
Maybe at that time, with Fiennes only starting out in his first full film as M it was too soon and therefore a bad idea.
But now, with the proper explanation, M being disillusioned with the way politicians are treating the 00-section, I would welcome him going his own way - and CraigBond discovering that this rogue behaviour he himself liked to indulge in can lead down a very dangerous path.
Then again, and I myself argued so many times that way, I don´t really want to see another personal angle in a CraigBond-film. I´d rather have him go out on a normal mission in a light adventure style.
However, SPECTRE and the treatment of Blofeld left so much to be desired that a return to Blofeld and the personal angle seems inevitable, even necessary.
It´s kind of a mess, really, and I am curious how EON will resolve this.