You can do lots more branching after the GLT initiative cleans things up. Signal priority on the B and C + all-door PoP boarding tames most of the dispatching chaos going into Kenmore. The new stretched low-floor cars tame most of the dwell time issues in the subway. The systemwide signal audit whacks a lot of speed restrictions that have been around since the PCC era but make no sense with cleanroomed rolling stock and modern technology giving the operator additional tools. Dispatching reform with more computer traffic management...and hopefully someday actual automated signaling...trims the fat and inflexibility from it being an error-overpadded strictly human-controlled operation.How much more branching can you do on the green line? The central subway is already super congested.
Remember: there used to be FIVE western branches (in addition to 1 remaining South End branch that lasted into the Riverside era)...daily supplemental short-turn service on the Arborway and Watertown lines throughout their service lives...and episodic trials of short-turns on the BC and Riverside lines when those still had their respective mid-line turnbacks at Nickerson Field and Reservoir. And Needham was supposed to be built in tandem with Riverside, so the D was built from Day 1 with higher-density signaling than it actually makes use of as a solo service.
The last 30 years of ops brainrot in the light rail division is not at all indicative of the Green Line's native throughput, because it used to do lots more. The places where it's failing now are attributed to very clear-cut ops inefficiencies. Take out the trash with all the surface traffic conflicts and dwell hell and rediscover the forgotten art of slotting and there's room off Kenmore for not only the moderate-load Needham Branch but also Urban Ring-North and Harvard Branch spurs off a BU Bridge subway extension.
The worst-case, most conservative scenario for GLT reforms still serves ample capacity for a Needham Branch. Best-case scenario where they optimize the shit out of everything: lots, lots more expansion.