I like the M idea, would have provided a different chemistry for new M and Bond.
Strong is too similar in age and class to Craig to make a convincing M in this current scenario.
At best M should be a father figure for Bond (making some sense of the orphan's loyalty). Fiennes doesn't really make this grade either (hence they had he had perform an act of bravery in SF to justify Craig's loyalty), but at least there's an obvious class divide between to two individuals allowing them to spark off of each other a little.
The worst scenario is that they are essentially the same and we're not convinced that Bond is in anyway subservient to M.
Obviously i'm working from Fleming's template, but he did do it rather well.
ETA: Personally i'd have gone for Michael Gambon, or Charles Dance as M. This would've really rooted the Franchise back in classic cannon/Fleming territory and afterall this indeed appears to be Eon's goal.
After all that misdirection in the run up to Skyfall, I rather like the idea of Ralph Fiennes as M. It will be a different dynamic between Bond and the boss next time because the age gap isn't that great, but Mallory has earned Bond's respect, not only by his act of heroism during the enquiry shootout, but his underplaying of his former role in the SAS ("Bit of a shock for someone unused to field work" - was there a slight reaction of "I know different" in Bond's face when the new M said that?)
As for the view that M should have a naval background, preferably as an Admiral, well we're in the year 2013, not 1953. These days it's as likely that the head of MI6 would have a background as a former station chief in Hong Kong, or an ex-special forces officer, as a former head of Naval Intelligence.