Are modern borders required? If not, if Mexico wins the Mexican-American war and retains all the territory it historically lost, that maybe it could be a contender if they can industrialize. Of course, all the gold from California would help. This affects the western settlement in the U.S.A. , but the rate of immigration to the U.S. would likely remain the same, which could lead to some larger cities emerging in the west sooner as people had less room to expand (or less people moving west as the historically people just skipped over the great plains region and west straight for the west coast mainly.) Canada is tougher one. In OTL Canada proved to be quite capable of defending itself, but the its main obsticle to becoming a great power is its, relatively, low population (modern Canada's population is one tenth of the U.S.A.'s). Hope this makes sense as I'm not really an expert on the era I just talked about. Of course, other POD's are possible.