We can go further in Canada - does it start from Europe, or from Asia, does it start from Confederation or from the War of Vimy Ridge? What about the new arrivals, and the not so new ethnic arrivals - what of the mass flood of Eastern Europeans, notably Ukranians into the Prairies?
The truth is, the bulk of the best texts of Canadian literature function to undermine ideas of Canadian identity and tradition as ethnically biased - there is no one voice that has ever managed to sustain itself. The only reason for the prevalence of Atwood is because in the 90s she sold out and became a "World author" abandoning most of her Canadian affiliations. And even then, she never even came close to representing anything other than Ontario.
The US is different, but as the above poster said, the one defining thing about Canada is that we don't want to be like the people to the south of us. Atwood once wrote something along the lines of "The difference between Americans and Canadians is that Americans think they are above of Mexico, and we know Americans are bellow us." And I think that holds rather well within the literary discourse.