As no one has rhymed in on Canada yet, I'll put my 2 cents in.
Canada really only has 3 big cities (Toronto, Montreal, Vancouver) so looking beyond those the obvious choice is Calgary. Both Alberta cities (Edmonton, Calgary) have been growing rapidly for most of the last 80 years. Only recently have they grown large enough to attract big city amenities.
They're already home to pro hockey and pro football. They're building LRT, new museums, 2nd universities, and have urban planning policies that encourage increased density. Where before they had lack lustre sleepy downtowns one can see the emergence of vibrant downtowns. They're wealthy, have young populations, exist is a pro business jurisdiction (Alberta), and have a can-do ambitious citizenry. I wouldn't be surprised to see MLS and/or NBA in one/both cities in the next 25 years.
Some may argue that they're oil and gas dependent but there are signs everywhere that both are slowly moving away from dependence on energy. Calgary has a large professional services sector that's looking for life beyond oil. The University of Alberta in Edmonton is a big research institution determined to move the province beyond resource extraction.
That said, I'd put my money on Calgary over Edmonton. It may not be the provincial capital but it's long been politically influential. Calgary and Vancouver are the big airport hubs in western Canada and seem early on in their growth trajectory. 3 million in metro Calgary within 25 years? Possibly.
Calgary, Alberta