The Green Line will never be extended to Needham Heights - the Needham Heights commuter rail line already goes there -no need for duplicate services.
An easier, but very expensive, option would be to replace the commuter rail service with Orange line service, but with the orange line running on a third rail they would have to switch to catenary cable past forest hills station and need all new cars. Both the Orange line and the Needham Heights services run at standing room only capacity during commutes, so the current system of parallel tracks doesn't seem quite so redundant.