For conventional elevated rail, there really aren't that many corridors that are wide enough.
The wide viaducts block out nearly all light from the streets below, which is made much worse if it's surrounded by tall buildings. Here's my list of streets from a previous discussion:
I'd probably also add the Grand Junction, and upgrade my evaluation on Broadway/Alford to be more positive.
If we're willing to consider a suspended railway, and I'm aware I'm really pushing the limits of a crazy transit pitch here, that would potentially open up more corridors.
Suspension railways have significantly less negative impact on the street below in terms of blocking light, and could in theory be used on any corridor wider than around 65ft, which is most main streets. I believe the SAFEGE and SIPEM systems have also mostly solved switching, so you don't need the slow sliding switches of a conventional monorail or the Schwebebahn. Stations still need to be much wider and therefore more thoughtfully placed, but it's a lot easier to find that kind of space every half-mile or so than a continuous corridor.
Of course, the big problem is that essentially zero existing infrastructure could be reused given that Boston currently has zero suspended monorails. (and I don't think any have been built since the last time I checked.) The swaying also presents a minor challenge for accessibility but that should be solvable.