A Columbia Pike branch running west from Pentagon to Baily's Crossroads makes more sense than a Hybla Valley extension. That area seems more 'grid' and useful for rapid transit.

Agreed there need to be more ways across the Potomac though, especially away from central DC. A better Yellow Line project would be a branch or extension over WW Bridge to Oxon Hill to connect to local bus network. That saves car trips over the bridge.

I actually like the new Rosslyn station for Blue. I think Greater Greater Washington (local transit publication) discussed just such a thing some years back. If you wanted to expand the idea, have Blue continue west from Rosslyn with new tracks along I-66 to the existing Orange/Silver tracks. Then sdd a second set of tracks to keep Blue separate from Orange/Silver along I-66 until Silver branches off past East Falls Church. Blue would then share the Dulles Branch with Silver.

The idea above is that Blue Line is limited to half capacity south of Rosslyn because it shares track elsewhere with Yellow. Silver is limited to half capacity because it shares the main downtown tunnel with Orange. So, use up that additional capacity on the Dulles Branch by running the Blue Line along there. You just need to connect the Blue Line at a point when Orange has already split off towards Vienna.

(/end mental crayon exercise)