Have you seen the
Green Line Reconfiguration thread? Lots of good stuff there.
Essentially you're describing what the Silver Line Washington Street should have been built as in the first place, usually referred to around here as the F-Line (sharing the moniker with a very knowledgeable and prolific member) along with the northwest quadrant of the Urban Ring (a mothballed official project, though the real-world version was proposed as BRT, which was the style at the time). [Since I'm not certain of your familiarity with those discussions here, I'm just summarizing for anyone who might not have read through the various threads on these topics.]
Both of those projects live somewhere in the grey area between Reasonable and Crazy transit pitches; there's nothing particularly difficult or unreasonable about either, but they're big enough and lacking enough in official support that we shouldn't expect to see them on the immediate horizon (though if the next governor is someone better on transportation than Baker, which is admittedly not hard, that could change, I suppose).
I'd be pretty surprised to see the service pattern go from Nubian through the Central Subway directly out to the Urban Ring/Everett/Chelsea. Depends on the line/terminal assignments, but the F-Line isn't going to be grade separated like the D-Line (I don't know if there's room for a reservation, but dedicated bus/LRV lanes at least approximate it). Maybe some of the cars would, but I suspect in an Urban Ring world we'd see a lot more use of my own namesake, the Brattle Loop, for cars from the Ring and north and a lot more cross-platform transfers than we do now.
