Restore originally-conceived line out Columbia Pike, eventually to Centreville via Braddock. Extend Silver line to Leesburg, Orange line to Gainesville. New line from Sterling to Potomac Mills via Manassas, with transfer at CIT and Centreville (and to Amtrak at Manassas). Extend Blue to Potomac Mills or even Quantico.
This is a long, long term plan, but needs to happen. Metro construction should never have stopped--it should keep growing for as long as the metro area is growing.