Rather than extending DC Metro, I'd build more branches along with infill stops in DC. Something like this. Assume the yellow lines are re-extended north and the blue is deinterlined with a rail shuttle between Rosslyn and Pentagon.
At this point you also probably want to drop colors as a naming scheme and switch to something else. I think I'd do number-letter: red from Potomac to Wheaton is A1, from Rockville to Calverton A2, etc.
