Sullivan's do-able as a ring route if it cannibalizes the freight siding (could also do away with the test track and reconfigure the station), winds around the industrial park and fringes of BET, goes through the first leg of the Grand Junction, and connects to the Green Line at an intermediate station at Brickbottom. The various ROW's are so wide there it's got good "superstation" possibilities.
But that would NOT be a part of the Union/West Medford extensions, unless the T wants to complicate and confuse those projects to a standstill and withstand the barrage of lawsuits from Somerville. Sullivan is strictly useful as an Urban Ring route via the Grand Junction. T wants to do this as BRT, which is an insane waste considering that it's a pre-existing RR ROW. But where that's going to make more sense is when you see MIT/BU/Harvard sync their respective political pressures on getting rapid transit up the Grand Junction and terminating at Harvard Square (already in Harvard's capital improvement plan, MIT has plans to bury Mass Ave. to make a stop there, and BU wants to build on Pike air rights to get a new central transfer station at BU Bridge). With that ROW already at 2-track width to the CSX yard and Harvard razing just about everything between there and the Stadium you will probably see that targeted as the next extension after all the ones with possible litigation (Red-Blue, Lynn, West Medford, Arborway, Dudley LRT replacement) reach some sort of final-final conclusion. Once you do that it gets pretty trivial to take it across that no-man's land of rail ROW's into Sullivan...and the various configurations you could run out that way to any which direction on the Green (especially with Brattle Loop in play) would make it a valuable link. Hell, the OLD Routes 92 & 93 trolleys that went on differing routes from Sullivan to Brattle Loop via North Station Under until '49 were tough losses.
But back on topic...keep it simple, damnit! Just get the trolleys rolling out past Lechmere first and then start dreaming. West Medford alone is not going to provide demand for s Sullivan link. It's the Urban Ring that's going to do that, and they shouldn't co-mingle projects until it's time if we ever want anything built. All I want them to do with West Medford that relates to future Ring or extension planning is to 1) construct the Union/West Medford ROW in the Brickbottom area intelligently enough that they have the option to tie in future crosstown trackage with the subway-fed trackage in intelligently-looped fashion that can allow multi-directional revenue routings and a provision for a Brickbottom transfer station, and 2) don't @#$% ruin a viable crosstown rapid-transit rail ROW by tilting at more BRT windmills. That's it. Just future-proof the space available at the big junction point by the shopping center, the McGrath Highway overpass, and the Brickbottom neighborhood before the whole area gets built up...and then stay the hell away until they've built what they've already (overdue) promised.
And I'd still want to see Union get extended to Porter before the Ring routes get built. That one's so easy as a next step it's a no-brainer: take land back from the industrial abutters who carved up what used to be a 4-track-width Fitchburg ROW on that narrower section from Union to Porter, and do some sort of station approach construction from the Beacon St. bridge to the headhouse to fit direct transfers for both lines (either by inclining Green to street level and covering the CR canyon with a grade-separated trolley stub-end, or lane-shifting Green into a cut-and-cover tunnel that dips under the last 500 or so feet of Somerville Ave. to terminate the line inside the station lobby). Voila!...nobody from Harvard or beyond ever needs to choke Park St. again en route to North Station or GC.