The only reason I had mentioned Greenland is because I thought it was interesting that apparently some of their own people seem to think that they'd be better off today as a US state (I agree with the assessment). I wasn't inferring that they would be annexed to the US anytime soon. I think their people want a resolution to their current status (much like the people of Puerto Rico), but they have a long, hard road to resolve it with Denmark. I would be in favor of Greenland as part of the union, but it's not something that I expect to happen, especially not anytime soon.
I just had a talk about US-Canada merging/annexing with my uber driver the other day and she brought up the good point of Canada still having a queen (and when she dies, a king) that reigns over them. That would ultimately hinder such an union. I kind of tend to agree with that. Curious what other folks think about this.
Regarding Mexico, I only see the Northern mexican states joining after some kind of recession or separation from those states. I also don't think the union would accept their request to join unless they cleaned up their place first. Furthermore I think a situation such as Mexico going into recession and selling states like Baja California, Baja California Sur, Sonora etc is a possibility. I'm not sure how much work it would take to get those states out of the slums and into, but we'll have to see what happens once Puerto Rico joins.
Speaking of PR, there's been some big movements happening recently. Puerto Ricans were in front of the White House demanding, Pedro Pierluisi has already spoken twice to congress requesting a resolution, and even wrote a letter to Obama. I believe another PR representative wrote a letter to Obama as well. We might see a federal binding "Yes or No" referendum on Puerto Rico that will determine the future of the island very soon.