While Canada, Australia and New Zealand are, geographically and politically, about as splendid as a country can be, I can't really see a niche for them in Civilization. They don't really have a very distinctively individual culture (apart from the cultures of the indigenous inhabitants) and have never had a period of global or, except perhaps Australia, regional dominance. I'm not saying periods of dominance are necessarily a good thing to try to achieve, but it's generally a prerequisite for inclusion in the civ list.
This is an outsider's view from the reviled mother country though...
This is an outsider's view from the reviled mother country though...