What should be rather easy and what wouldn't change Canada that much is to have the western border of Canada being more in the South, thus Michigan, Minnesota, Wisconsin, the Dacotas, Oregon, Washington, Idaho and Montana end up in Canada.
Then increase immigration - for example by allowing all those ethnicities that the US didn't.
Finally, ITTL getting Alaska and Hawaii into Canada is possible.
The added landmass brings some millions of inhabitants into Canada, increased immigration can easily provide further millions, so Canada ends up with 100+ million inhabitants and certainly is a world power by then.
Then increase immigration - for example by allowing all those ethnicities that the US didn't.
Finally, ITTL getting Alaska and Hawaii into Canada is possible.
The added landmass brings some millions of inhabitants into Canada, increased immigration can easily provide further millions, so Canada ends up with 100+ million inhabitants and certainly is a world power by then.