This is very difficult with a 20th century PoD.
The best way to achieve this is to avoid the Great War. Immigration was exploding in the lead up to the war, and even it tapers off somewhat for the following decade, you're looking at millions of extra people by the 21st century.
I'm always leery of the Mid-Canada corridor, because I think it's mostly an economic anchor that wouldn't provide a ton of extra value and would trap people in towns with little economic outlook and depressed property values.
I'd recommend just totally butterflying away Saskatchewan and keeping it united with Alberta as the province of Buffalo. Saskatchewan is too poor and thinly populated to have properly exploited its resources, and Alberta consumed all of Saskatchewan's growth for seven decades anyways. If the capital remains in Battlefird, which makes sense as a central hub, the province likely has three major cities: Battleford, Edmonton, and Calgary. Edmonton may not be be as big as OTL, maybe 50% of the size, but Battleford is probably also hovering around a million people. Swift Current, Moose Jaw, Weyburn and Estevan are probably double their size too. Regina is likely butterflied away. If oilbis discovered ten years earlier, and there's no reason it couldn't be (it was government issued exploration credits than technical feasibility) you'll see the oil boom kick off a decade faster than OTL and give another decade of growth, probably adding at least half a million souls to the province and some much needed currency during the Great Depression.
The Mackenzie Valley Pipeline being approved would do absolute wonders for northern development and could easily see the territories double the OTL population from all the investment making it easier for other economic activity to tale place. Two extra pipelines to tidewater keep the WCS/WTI spread low and keep Canada's oil more profitable, driving further long term growth. Not using the NEP adds another $500 billion+ to the Buffalo GDP too, adding even more people to the region.
I'm still leery of a big and strong province being able to build a road all the way to Uranium City. From the west is impossible because it's nothing but a giant inland delta, and from the east you need to build about 200 bridges along the shore of Lake Athabaska. There's nothing saying it can't be done, but the cost would be astronomical. That said, a strong province would likely see a lot more investment because it wouldn't let all these projects languish in obscurity from a lack of government resources. Alberta always had far more economic dynamo than Saskatchewan regarding infrastructure investment and business tax credits.
The Georgian Bay canal is just too late for a 20th century POD. There's no need for it when the St. Lawrence Seaway gets built. You're spending huge sums of money to divert a small amount of goods slightly faster. In the mid 19th century it might have made sense (not that Canada could have afforded it), but not in the 20th.
Buffalo is an interesting idea, I like where you're going with that. Was that something being considered at the time in 1905 (and was that name being considered for the "united" new province)?
I've also been excluding most of the far north stuff because of the costs involved. GBSC maybe could have happened in the late '10s and early '20s if things had gone differently? I know it's a reach and wildly expensive for not a tremendous amount of gain outside the local region. I agree 19th century Canada likely didn't have the money to build it, while 20th century Canada probably lacked a reason. But are there any divergences that would make the UK interested in funding some of it/increase pressure and reason for the canal at any point?
Also what's the "NEP"? Not familiar with that acronym. Also "WCS/WTI"?
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