East Somerville Station, and the site of the proposed Route 16 stop, have the most space it seems like for an additional CR stop. East Somerville Station is where the CR branches off, but there there's no rail connection to Union Sq. (a bus connection is still available, but the 86 bus is only every 35 - 40 mins frequency)
Otherwise, Route 16 Stop seems to be the other option, since it's surrounded by parking lots on both sides of the ROW, but both the GLX would need that extension to Route 16 and the CR would be right at the bridge, so...
Other Medford Branch stops either do not have sufficent ROW space, and/or the bridges would need to be rebuilt again.
For now, To get between the GLX and the West Medford CR station, the 94 bus is currently the main way to make the connection, aside from travelling down to North Station. The 80 bus is also available, although I'm not sure if the MBTA is going to cut the 80 back to College Ave. in the winter 2023 quarterly schedule adjustment, reduce frequency, or if it's going to wait until BNRD Phase 1 in 2023. Stay tuned for the winter 2023 bus schedule changes coming soon-ish.
I don't think a CR station at College Ave. makes the most sense, since the entire walkshed is 100% dominated by Tufts, and when school is not in session, ridership would be extremely low. Even with the GLX, GLX is opening right at the start of winter break, so ridership at the College Ave. GLX stop will be entirely reliant on school being in session or not.