I don't see that happening anytime soon. Dedicated bus lanes, signal priority and all-door boarding are more realistic and can get done now, with a little bit of paint, a little bit of electronics, and a little change in policy.
The 66 as well. Plus remove some of the obnoxious route jogs (that's a topic Jarrett has often emphasized).
The MBTA's track record on all three of those things is bad, bad, and worse in that order. I'd press for a Mass Ave. Subway precisely because setting an unrealistic goal might cause the MBTA to 'compromise' down to the necessary improvements you've listed.
As for the 66... I don't think its problems are so easily solved without pouring concrete somewhere
. Brighton Avenue is a complete disaster and no amount of paint, electronics, or policy change will fix that, especially the stretch between Union Square Allston and Packard's Corner.
The 'easiest' fix for it is likely an exercise in relative speaking - there's no way the 66's problems are addressed without a comprehensive solution.
I maintain that nothing can be done about that road for as long as it's emphasized as a major thoroughfare - the US-20 bannering needs to be moved somewhere else. I've been assured by some smart people that pushing it onto Leo Birmingham/Soliders Field/Storrow is a bad idea, but I'm pretty sure that's the only place it can realistically go unless MassDOT's willing to concurrency it with 95/128 at the current interchange and run it onto the Mass Pike that way, or concurrency it with the Pike from as far back as the Millbury interchange. Why is this important? Because, as long as you've bannered it as a highway, people are going to treat it like a highway, and no amount of traffic calming is going to change that.
So, once that's done, we can talk about the actual transit improvements to be made. Resurrecting the A Branch would probably take several miracles in rapid succession at this point, and I'm guessing that also applies to a phased restoration that starts by restoring Packard's Corner - Union Square and running trains from Allston to Lechmere. With that out, the next best option is reconfiguring the stretch of road between Packard's Corner and Union Square entirely. West of Harvard Ave, we have five - maybe six - lanes worth of space to play with, so it shouldn't be too difficult to turn the rightmost lane on each side into bus lanes, make the center lane the only through lane and leave the center two lanes as either a wide median with cuts for left-turning lanes or (loathe as I am to suggest one) a cycle track. In either case, take Union Square and make it into a bus plaza. Harvard Avenue itself should be made one-way southbound, with Linden Street remaining one-way northbound. (In each case, knock it down to two roads, through traffic on the left, right turns only on the right.) Then, disallow movement between Cambridge Street and Brighton Avenue by any means necessary. Buses and pedestrians can use the new Union Square plaza to move from one to the other, and cars can use the one-way street pair. Nobody in any case should be making that turn at the intersection, and BPD is going to love the increased revenue from parking an officer or two at Union Square and hitting everyone with a violation for Illegal Turn until people get the message.
Of course, that's slightly less relevant to the 66 because the next thing that should be done is to move it off of Cambridge Street and onto Everett Street instead - hitting New Brighton Landing if possible, and following Everett Street up to Western Avenue, over to N Harvard Street, and terminating at Harvard. (Could be worthwhile to through-run these buses as 66 -> 69 to Lechmere.) That's the most important operational change to make.