Assming "normal scheduled operations" to include July 4th mix-ups, the following then have no business in the codes, unless WMATA has plans for using these in the near future...
Union Station
Twinbrook
Fort Totten (Red, Yellow, Blue)
Rhode Island Avenue
Van Ness-UDC
Farragut North
Medical Center
Ballston
Greenbelt (Yellow, Blue)
Huntington (Blue, Green)
National Airport (Green, Yellow)
Arlington Cemetery (unless they allow passengers on July 4th to ride the Rosslyn Blue trips that far)
New Carrollton (Blue)
Largo (Orange)
Franconia-Springfield (Green)
Anacostia
Stadium-Armory (Yellow)
Smithsonian
Federal Center SW
Dulles as blank seems like it could be used for any of the shared four... I'd imagine the blanks from 68-70 and 76-79 are for the White/Silver line's turnbacks. Those remaining in the middle of other occupied destinations are probably for whatever line is closest... If Red ever expands, then it'd probably get anything created numbered 80 and higher.