".. aren't interested in being shoe-horned into Virginia or Maryland.

Some wouldn't care, some would. As I've said elsewhere, I personally would favor either making the District just the Capitol Hill area and making the rest Maryland, OR expanding DC to the original rectangle that included Northern VA.

Thing is, there will be vehement opposition to all of the alternatives..."It all depends on whose ox is being gored."
I don't think much will actually happen.

I think I would support Puerto Rico being #51... all the other 'possessions' are too far-flung to include any of them fairly.