The Washington Post ran a story earlier this month announcing the opening of an AC Hotel in National Harbor this spring. (Marriott to debut new boutique brand in National Harbor - The Washington Post)
This will join the recently opened AC Hotel New Orleans and others under construction in Miami, Kansas City, and Chicago. I know tryt53 stayed at the New Orleans property and was looking for the option to write a review (When Will We Be Able to Review AC Hotels). More discussion of the AC brand can be found at this thread: (Re: Marriott debuts hotel brand for Millennials).
Overall, the National Harbor area is beautiful and it will be nice to have another Marriott option there.