Actually the Continental Congress (I don't remember which one) sent a delegation to Quebec to ask them to join the revolution, just make the delegation more convincing somehow. Then I think the two largest questions would be what happens to the rest of British Canada, does the US eventually annex it? Then if Quebec manages to hold on to its identity (which I think is a good possibility, but I'm biased here) the Civil War could be an even bigger mess as Quebec would defiantly be for greater states rights (so they could keep their identity) but also probably be against slavery.