How about a campaign where Quebec rebels against the Canadian government and tries to declare independence. It could start out with the Quebec Army building up in a border town of Ville-Marie and then one night they just flood over the border, capturing an unsuspecting Ottawa. At the same times, troops in the East roll through Newfoundland, New Brunswick, and Nova Scotia to give Quebec complete control of the East.
Now you have to either play as part of the Quebec army and continue to fight the Canadians as you expand. Or you play as the Canadian Army and attempt to recapture Ottawa as well as the three provinces lost in the initial invasion before Quebec takes any more land. Of course you can have them keep advancing for a little bit at the beginning. Maybe they continue south and capture Toronto and then eventually take the entire Ontario peninsula and begin to invade Manitoba before the Canadian Army gets an adequate assault team in action.
And you could decide to have the Americans join the war on either side, or maybe they see it as an advantage to gain more land and attack Montreal and then try to expand from there to take as much land as they can get before someone finally stops them. Lots of things could happen.