Your Green line branch to Medford center is (currently) a waste of resources. Past Assembly, Mystic ave is barren outside of industrial, car dealerships, and some scant food establishments. I get it, because it's a wide open space that gets you to Medford center, but in my opinion, it's no better than the Northern Strand because 50% of its catchment is the river. In this case it would be the highway and the river. According to the better bus data on the 95 bus, there are just 400 riders Mystic ave down into Sullivan.
Now, could the area be redeveloped into a more functional (read: density) main road? Maybe? But that is squarely flood zone on both sides. One could argue that if Wollaston can get the red line, Mystic Ave should be approachable for the Green, but Wollaston was built years ago and the grade separation is above the flood zone. Street running trolley would be different as would developers willing to build into a flood zone given the discussions around climate change.
I would shore up bus service with lanes to boot and leave it at that in perpetuity. Given this is god mode, my solution for Medford center would be to take Fellsway north and then tunnel under the former right of way into Medford center (the branching orange line that people seem to favor).