I'd love to see Colin Firth in a Bond film in any role. I agree he'd be great as M though.
Before Craig was cast I suggest Firth for Bond himself. I forget who, but several people said I was crazy. I still stand by that. Hey might not be the much uber action star, but dollars to donuts he could do "spy" better than the whole lot--including Connery. Plus I just really like him and always have.
Actually, if they're going to plop him someplace, why not Tanner? I've always said he needed to be involved in the films more. The 4(?) to date have been rather underwhelming.
And Bettany still has my #1 for Q.