Disclaimer: I am Canadian, so I am very biased in that regard.
I don't see a world where this ends positively for American citizens. There is a few different outcomes, but none where the positives outweigh the negatives.
1: Trump continues to engage in a trade war, eroding trust with USA's second biggest trading partner. This will damage both countries economies, although the USA will sustain far less damage. But the USA will have their global reputation further impacted, Canada is a respected country with strong allies, all for the next president to most likely back away from this.
2: Trump somehow convinces Canada to secede and become a state. This is the most unlikely outcome, as Canada doesn't want this, nor does Trump. There are undoubtably Canadians who would vote for republican/MAGA politicians, but it is not a large number by any means. If equitable voting was granted, it would be like adding another California to your federal elections.
3: Trump invades Canada/Annexation. This would be the dumbest war to wage. The border is unmanageable and Canada would fall under military invasion. But the US economy would plummet, countless American and Canadians would die, and the USA would become isolated globally.
I don't know why this is wanted by American citizens. No matter what happens, it is the average American citizen who would be paying the price for this. It just seems like another way the Trump administration is making the average American life harder, while pretending it will improve it.
EDIT: This is not a post about who will have it worse between Canada or the USA. Canada will, I thought I was obvious in that. This is about how any of this actually benefits Americans.
And regarding people commenting that Trump is joking around with the 51st state stuff. He is your president, what he says holds weight.