I wish there were a way to make a subway start-up. And for cut and cover [1] to be practical. Really the only way I could see this getting built:
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Split the red line at central, go down mass ave with connections to the green and orange lines, and meet back up at jfk/umass. See it on a larger map with comments. It would replace the number 1 bus, and take a lot of load off the central subway.

[1] The oldest subways were built by digging up a street and then making the tunnel by putting the street back over it. Quite cheap, quite simple, quite disruptive. Most recent subways were built with deep bore tunnels, which are far more expensive. Aside from the disruption, there's also a lot more stuff (cables, pipes) already there, and these would have to be moved.