What if in the War of 1812, Britain annexed a larger chunk of Louisiana and the old northwest?
Below, I have a border set at the 42nd parallel from Lake Michigan to the Missouri River (with the exception of a triangle of territory between the Mississippi and Des Moines Rivers) and the 43rd parallel from the Missouri River to the continental divide. Later on, Britain gets more of Oregon Country and I give the British Northern Maine because why not.
One big advantage is Canada will have a more southern railroad route west via the Michigan Upper Peninsula and will have the resources of northern Minnesota and northern Michigan. There would be more agricultural lands as well, particularly east of the Missouri and out in the pacific northwest.
Canada would probably focus a lot more on settling those western regions than the US did (think of the Last Best West campaign of OTL).
Many TLs that give Canada more of the US north include the US getting more of Mexico to balance things out, but given how the main goal of expanding west was California ports I'm not quite sure what the advantage in that would be. Maybe there'd be a desire to settle the Texas border further west in 1819 but that's different.
I imagine many Yankee settlers who bolstered the early Republican party OTL in western states (Wisconsin, Iowa, and Minnesota especially) would still be going west in the same manner. Maybe a few more would settle a bit further south in order to stay in the US, but on the whole the abolition-minded will be a smaller group in the US most likely.