According to Steve Goff there are a couple of new sites that might be up for consideration as a location for a DC United Stadium, Buzzard's Point and Capital City Market.
Buzzard's Point a semi-industrial area just south and west of the Washington Nationals baseball stadium. There appears a cement factory and other industrial spaces to be unused space sufficient to house a 20-25,000 seat stadium, at least one training field (perhaps 2) and some parking facilities. It is not exactly mass transit friendly in the immediate area (the nearest Metro stations are about 6 blocks away, which is a fairly long walk. But interestingly, across the Anacostia River is Anacostia park. The space could also be re-developed to a "river front" space might bring some more money and could have some harbor facilities for water taxis and other water born traffic to the stadium.
Capital City Market is a area much closer to mass transit, two or three Metro stations nearby depending on how far you want to walk and the location and rail through Union Station, as well as general proximity to buses and major road transit. DC has been slowing developing the area just north east of Union Station with the Bureau of Alcohol, Tobacco and Firearms putting a new headquarters building nearby, Galludet University, Fed Ex facility and other business/tourism sites not far away. The more difficult problem the most likely space is currently in use and would require the condemnation of some already commercially used property, but the facility may be able to hold a 20-25,000 seat stadium (with room to expand) and a couple of training sites.
Given that the City of Baltimore has made a play to put a stadium in Charm City not far from M&T Bank Stadium (which has hosted some high profile international friendlies), DC has finally sat up and taken notice. Baltimore hates the fact that they get ignored in the proximity of DC and the DC city council probably couldn't stand losing DC United to Baltimore (the suburbs maybe like Fed Ex Field), but not Baltimore City. such a prospect might just gall DC United and the City Council to get a deal done. But I am not holding my breath.