The inherent problem with Metro is that it's a compromise of rapid transit subway and commuter rail. You can't overcome exurban sprawl (which is inherently far from walkable urban centers by design) by shaving off a few stops.

Ashburn-upon-Dulles is best served by commuter rail, which, short of a third Metro track, could probably be returned over the old line from Leesburg that could also pickup other communities currently off Metro.