While I like the idea, it could bring weekend service to Stoughton, the problem is it'd eliminate the Stoughton Branch's pretty unbeatable time to South Station. It's only 40min to South Station and 30 to Ruggles. The only way driving is matching that is in the dead of night. Tacking on the Fairmount to that throws an additional 30min on the journey, making it take as much time as coming from Providence and losing its reason for so many commuters to take it. You could time it for Providence Line transfers at Canton Junction but then you'd lose both the doubled-up Canton/128 headways into Boston via the NEC and the Providence Line is already at passenger capacity during peak.
If we had the proposed Stoughton electrification and 15min service to key stations, I think the next logical step would be to diversify the trips to bring more utility to train travel. Have some Stoughton Line trips run via Fairmount say hourly, some Providence Line trips run via Foxboro and Dedham, a daily Worcester-New Bedford round trip via Back Bay skipping South Station and seasonally to the Cape, so on and so forth. Truly connect our state via rail. These are all possible with the current infrastructure, entirely within MBTA-owned rails, they just require investment, personnel, and initiative so as not to detract from any existing service to the primary commuting demographic.