Looking at that map, is it possible to extend the union square line further (say, 2120) or is there a wall in the way?
Also, the Washington DC map is exactly what I mean, although it would work better in Boston. The map posted is pretty confusing.
San Francisco is another example
My proposal would have the two colors sharing a line only for 5 stations though, Gvt, Hay, North, Science, Leachmere.
The reason I think the E branch would be ok to keep from heath to union is because it seperates itself at Copley, and follows a similar path as the b/c/d branches (in how a map looks).
The F branch for me would be something like the washington street line leaving Boylston.
I hope the MBTA doesnt screw up how they name the new branch/line