Of course, anything is possible, but I would be shocked beyond words if this were ever to happen. I do not envision any way in which either side would be happy with such a merger.
On the Canadian side, my impression is that a major part of Canadian identity centers around the idea that they are not the United States. For example, Canadian students sometimes post questions on eNotes about things like Canada’s particular role in peacekeeping since World War II. These questions appear to me to be meant to present a contrast toward the US’s more aggressive actions. I would also think that Quebec in particular would be very unhappy about being put into a still larger English-speaking country. They would fear the loss of their identity even more than they do now.
In today’s polarized America, I can’t imagine people accepting a merger with Canada. I think that conservatives, in particular, would feel that adding Canada would dilute American identity. Conservatives would fear that the addition of Canada would lead to what they see as European-style socialism as the former Canadians would support things like a single-payer health care system.
Finally, I don’t really see any strong incentive for either side to merge. It is not as if it would be a great benefit to either side.