Congress will not admit Puerto Rico as a state until the island becomes majority English speaking. If Puerto Rico became independent, most Americans wouldn't care. The island seems satisfied with its current commonwealth status. The Virgin Islands will not become a state because it is too small, and the people there are unlikely to agree to become incorporated by Puerto Rico as its much greater population will de facto take away any control they have over their own lives. Any of the Pacific territories are likewise too small and too geographically far away.
There are various movements in multiple states to split them into separate states. None of them enjoy broad based support and are fringe movements. Still, historically it is possible to have created such states, or determine different state boundaries. As far as I know, there is no movement to combine existing states into a merged state.
There is no possibility of any foreign country entering the union. The only real candidate would be the provinces of Canada, and that is unlikely to happen for the next hundred years or so. There would probably be much soft support for such a thing among Americans (meaning they would welcome it), but very little hard support (meaning active advocates). And among Canadians, there is a very strong population that would be against it with a much smaller group being indifferent, and an even smaller group that would welcome it. However, I think eventually some kind of Canadian entry into the Union (outside Quebec) is probably inevitable given historical processes.