Here is a crazy thought about Ralph Fiennes character. Consider he is playing some sort of "government agent" and the movie has something to do with "secrets from M's past", what if he plays M's son (or other relative) who is, presumably, somehow involved with the trouble that occurs in the film?
I could see why they would want to keep the role quiet for now considering that is quite the addition to Bond lore (for lack of a better word). I'm probably wrong, but I'm going to run with that theory for now. Hell, it is a lot more interesting than the "Fiennes is the new M" rumor.