For this map, I'd like to see the Purple Line just be extended for the full loop to make the rail equivalent to the Beltway. Connect all the major suburban DC locations together and connect the ends of the Metro lines. You can finally travel from Alexandria to National Harbor without having to take the Metro through DC and take a bus transfer.
Also, for good measure, I added rail extensions going towards Fairfax County, Southern Maryland, and Annapolis. The goal is to make the area easier to traverse without a vehicle and encourage more TOD in the suburbs to boost housing options in the expensive region.