In the extraordinarily unlikely event that y’all Canadians come to your senses and accept our superior American culture/the Canadian hordes come swarming down across the border to conquer and rule/the U.N. Commission on Boundary Rectification rationalizes the political boundaries of North America/whatever, I can’t really see why we would do anything but accept each province as a state. (The territories would presumably continue to be territories.) On the one hand, Ontario would only be about the sixth most populous state in the newly enlarged U.S., Quebec would be way down at number 13 or so, and certainly no other province even comes close to needing to be split up purely on population grounds. On the other hand, it’s deeply ingrained in our Constitution that state = equal representation in the Senate (to the point that officially speaking even a Constitutional amendment can’t deprive a state of equal representation in the Senate); and since the Canadian provinces have each been distinct political entities for many years it doesn’t really seem likely any of them would be forced to merge. The only one whose population would be seriously out of line would be Prince Edward Island, which is markedly smaller than the current least populous U.S. state.
At least many of these issues are largely determined at the state level, so it’s not as if Ontario would necessarily have to accept Mississippi’s idea on, say, gun laws any more than Illinois does.