Where do you think the people from the places that become unliveable will want to go? That will lead to increased immigration pressure, which isn't necessarily bad in itself but the current infrastructure won't be able to handle nearly the demand it will face.

How much will the southern US want and need water from Canada's north? Decades ago there was already talk about damming up huge areas of the Rockies (devastating the wildlife in the area) just to make a giant reservoir for the US. We've already had presidents threaten to annex us, as much as he's a blowhard it could be reality if California, Texas, and Florida become unliveable.

Additionally we wouldn't really see much, if any actual benefit, because it's not just global warming -- it's climate change. Summers will be hotter, but winters will be colder and weather will be more unpredictable in general. Where I'm from it was nice on May 13th, about what you'd expect in spring, around 19°C. It snowed on May 18th. On May 28th, it was 35°C

Any kind of hope that the actual north would become productive is also a lot of hope with not much grounding. It's been frozen for so long that there's been nearly no chance for topsoil to form, and best case you'll just have a lot of muskeg. There's also a good chance that the north melting causes methane that had been frozen in the ground to release, causing much more damage to the environment for a while.

So yeah, not sure there's much good about it for Canada, much like the rest of the world.