Edit: Reading the 1965 plan it looks like they recommended connecting the Riverside line to Huntington Ave as well! Get on that, Boston.
The Huntington-Brookline Village-Highland Branch to Riverside plans goes back even further to the 1926 "Report on Improved Transportation Facilities in Boston". The proposed alignment back then was a subway extension from the Pleasant St, portaling up near Buckingham St to follow the NYNY&H main to roughly Mass Ave x Huntington where the rapid transit would spur off to Huntington and lead out into surface tracks from Symphony to a terminal on Tremont St in Roxbury. The hop-over to Brookline Village and the Highland branch was proposed as a "future" ext.
EDIT: A lot of the proposals were meant to be redundant (e.g. Fairmount and main line rapid transit to HP), it's not like the Commission recommended building all of them - clearly, since so little was actually built.
There's a lot in this report too - I think 2030 and MassDOT could do a lot of good in just going back through the various reports from '26, '45, '66, '69, '72, '78 to see what's been planned (if an area could accommodate rapid transit even a 100 years ago, there's a good case that the "bones" of the neighborhood are good enough to do so today as well). Some of the other rapid transit gems from the report (while many are no longer possible, sure) are: Western Route to Reading/Wakefield with a spur to Medford Center, NH main to the Woob via Winchester with a spur at the old Central Mass division to run two branches to Bedford via Arlington and Lexington and a spur to Waltham via Belmont, ext of the old Charlestown El via the Saugus branch to Saugus and further, two proposed alignments to reach Dedham - extension of the Roxbury branch to Dedham via Needham (which you've discussed ad infinitum here) and extension of a rapid transit along the NYNH&H main (NEC) from FH to Dedham via HP and Readville, a rapid transit conversion of the NYNH&H midland division (aka Fairmount), and an extension to Braintree along the Old Colony (which actually happened a lifetime later).
The report also connected the Western Route rapid transit with the Fairmount via North Union and South Stations and studied a circumferential route, though didn't come up with an alignment (which Boston had asked prior in 1923). 2030 has a
a lot of digging to do.