I foresee a future where the United States of America undergoes a new era of expansionism. These are the territories I suspect will be on the US's radar as expansionism takes place:

Greenland: Yes, there will be a round two of trying to obtain Greenland. Ultimately, the Netherlands ain't doing anything with it

Western Canada: I suspect the WEXIT Movement will only grow after the Canadian corruption exposes itself. Because of this reason the four provinces of Western Canada will further experience "western alienation" and ultimately side with the United States.

The Canadian Territories: For the same reasons above. Canada ain't doing anything with it and they have no representation in the Canadian government.

Northern Mexico: Mexico will eventually collapse into crisis and the Northern region of Mexico, which is already closer to the US than to the Mexican Capital, will ultimately leave the nation and join the US.

Several Caribbean Nations: After Puerto Rico gains their statehood, I imagine that the people of several Caribbean nations would become more open to joining the United States for more freedoms, a better standard of living, and less corruption.

NOTE: I am very familiar with the Puerto Rican people. They WILL become a state. Also, I believe the US could expand further Southwards, but I would be against it as we'd be better off in first assimilating the new Latin American territories into our culture before admitting newer territories.