If it ever happens it will be Puerto Rico. I think we'll see some kind of new status for PR within the next decade or two. Whether or not it ends up being statehood is still up in the air a bit.
I don't think DC will ever be an actual state. There may be amendments in the future allowing it more independence and/or representation though.
It should be merged with Maryland.
That makes historic sense but I doubt there will ever be the combined political will within DC, Maryland, and Congress to actually make it happen. Maryland will never want it back and DC has grown too accustomed to being its own entity.
I think the most realistic "final scenario" at the federal level would be DC with a voting rep, two Senators, and 3 EVS. Locally, DC would get out from under the thumb of Congress for most home rule issues while Congress would still maintain major control over security and aesthetics. I don't think statehood will ever happen unless the portions of the great DC area are also included.