Back to the future: the original 1960's State plan was to have an interstate-standard expressway from Greenfield to the Inner Belt Expressway in Cambridge. If it were extended west to Albany NY, it could be designated I-92.
The mythology between various "I-92's" is fascinating reading. That's also why NH 101 is a mostly grade-separaded Super-Two west of Manchester, and why US 4 from Rutland to the NY border is Vermont's only non-interstate divided expressway. I know there was very briefly in the 'proto'-Interstate era a proposal to extend CT/MA 15 Wilbur Cross Highway (future I-84/I-86) from Sturbridge to Westminster on the carriageway of curiously grade-separated MA 49 with new construction paralleling MA 31 the rest of the way up. And that would've later tied into some of the plans (alternately proposed for 15 or I-190) to continue to Manchester hitting NH 101 on the Super-Two's north-south bent around Milford. I guess in that case "I-86 East - Boston" either would've bent along 2 inbound of Fitchburg to keep its control city accuracy, or the FHA would've had to go fishing for another odd-numbered 2-digit designation if it went to Manchester.
I know the MA 2 Super-Two west of US 202 in Phillipston was always provisioned to be doubled up into 4 lanes/2 carriageways someday. I don't know what the hell is up with the 12-mile expressway gap between Orange and Greenfield, however. There aren't enough people out there for that to have been a highway revolt, as Millers Falls is the only pocket of density that would've had to be accommodated. Probable path from where it would've dipped/dived around the current at-grade 2A concurrency around population pockets looks easily traceable as utility ROW's. I know that's absolutely no one's idea of a priority today, but that seems like it should've been properly finished and badged I-491 the whole way in to Alewife as the signature Boston-Vermont expressway. I definitely don't think the western leg through the Berkshires would've ever worked as an expressway. The grades east of North Adams are way too severe, the population even thinner, and unlike the Boston-Vermont connectivity there's not a whole lot of corridor juice way out west that isn't ultimately duplicating of the Pike to Albany. Pretty much all of the "I-92's" of legend were flimsy on-spec and driven by a lot of hot air from pork-hungry VT/NH/ME Congressional delegations. 2-begat-92 would've definitely been one of the flimsiest of all.
Definitely the grade separation of the remaining three substandard segments--128 to Bedford Rd., Lincoln (2 mi.), MA 126 to MA 111, Concord-Acton (6 mi.), and MA 12 to MA 31 (4 mi.), Leominster--are reasonable finishes. Lincoln badly wants the 128 separation, the Leominster curb cuts are barely any more numerous than the Lincoln's, and the long-underfunded Concord Rotary project stalled in prelim design does the lion's share of heavy-lifting for Concord by knocking out the most-complex 1.2 mile midsection of rotary approach to MA 62 (leaving just mop-up on either side). Get the big Concord Rotary project over the finish line and it can be a 55 MPH modern-design expressway from Lake St. to Phillipston without much additional effort. As for the western Super-Two, start first with doubling up the US 202 concurrency between Phillipston and eastern Orange, then finish the rest when the western Orange-Greenfield expressway gap is filled. Always seemed like a dumb idea to drop lanes just as the highway was picking up a major route concurrency, though obviously the traffic levels don't suffer much from it out there.
I'm against the idea of making this an "Intermodalway" corridor with the Fitchburg Main, however. Service past Fitchburg to Greenfield ran until '58, Athol till '60. Pre-'58 there was also thru-routing north of Greenfield to White River Jct., VT and Albany-Rennsalear via B&M's Troy Branch. Per the 1954 schedule
they threw a bunch of expressing darts at the wall and netted incomprehensibly broad travel times to/from North Station-Greenfield as low as 2:15 to as high as 3:40 depending on how severe the skip-stopping was. And that was at historical bests for Fitchburg Div. speeds, as the ROW west of the steep Westminster-Ashburnham grade is hard-bolted to the banks of the ultra-twisty Millers River and simply can't sustain speeds from one S-curve to another to another the whole way cross-state. The NNEIRI Study's Preferred Alt. for the Boston-Montreal direct train makes South Station-Greenfield on the L-shaped routing through Worcester & Springfield in 3:15...shorter on the built alts. that did 90 MPH track Palmer-Springfield and on the Conn River Line's Northampton-Greenfield straightaway. Catchment is no contest going South Station, Framingham, Worcester, Palmer, Springfield, Holyoke, Northampton, Greenfield vs. a Fitchburg Line super-express that's got to be trimmed back with some degree of brutality (Porter, Waltham, 128, Concord, South Acton, Ayer, North Leominster, Fitchburg, Wachusett, Gardner, Athol, Orange, Millers Falls, Greenfield???) to make a schedule time anyone will bother to ride. And let's face it...Athol (pop. 11.5K) and Orange (pop. 7.8K) are nobody's idea of sweetest catches worth setting stop spacing limits for on a super-express that's going to have to pass up lots of intermediate stops.
There's not a whole lot of upside there even in God Mode. But in Practical Mode where the state's own Admin. isn't trying to poison NNEIRI with the East-West tankapalooza, we'd be proceeding with the BOS-MTL base buildout that achieves this very one-seat at least once per day (2x de facto with SPG transfers). Then doing follow-on upgrades to the L-shaped route after it gets its ridership legs to bring the 90 MPH stretches online, add VTrans' desired third frequency (likewise hot-linkable to BOS from a SPG transfer) from NNEIRI, and eventually go fishing for an additional BOS direct when demand is cresting. 3 hours in Amtrak livery beating the snot out of losing all feeling in your ass trying to do the same with T livery on the Fitchburg Line, with all that Worcester/Springfield patronage goosing the farebox recovery FAR above what the northern route would. I'm not even sure the once-preferred B&M North Station-Montreal routing up the the Cheshire Branch (tip of Ashburnham horseshoe to Bellows Falls via Keene) would be enough of an improvement today with an NHDOT de-landbanking and quid pro quo's with Vermont Rail System for a revenue-underwriting freight lane into MA. Repeat passes at tightening the performance bolts on the L-shaped SPG Hub route simply rates good enough on schedule while hauling massively larger urban & transfer catchments.