Greater Greater Washington ran a post a few days ago about the possibility of a second Rosslyn metro station for the Blue Line, as a potential precursor for a separate Blue Line across the Potomac into DC. This led to a whole bunch of other ideas to improve Blue Line service (which has really been hit hard by the almost-open Silver Line) that would theoretically cost less. Among the ideas pitched are to convert the Yellow Line into a "shuttle" between Huntington and King St or to reroute the Yellow Line to Rosslyn, in order to route more Blue Line trains across the Potomac on the existing Yellow Line bridge.
The thing that gets me with these is that there's actually more people getting on/off at Huntington than there are at Franconia-Springfield. While there's slightly more ridership on the Blue vs. the Yellow south of King St today (thanks to Van Dorn park-and-ride commuters), redevelopment plans at Huntington and Eisenhower vastly exceed anything at Franconia-Springfield and Van Dorn. So it won't be long (maybe 5-10 years) before the Yellow Line has more ridership south of King St than the Blue Line.
In short, a Yellow Line "shuttle" is not a realistic option. Nor is terminating a direct connection from Huntington into DC.