I've long felt that Blue Line to Kenmore should be under Beacon if possible to gain more land in the walkshed and shrink the "swimshed."
Technically possible? Probably. Economically/politically feasible? Not a chance.
Blue Line to Kenmore via Storrow, taking the existing Storrow eastbound tunnel as part of the project, is mostly disruptive to Storrow itself, which isn't particularly a bad thing, seeing how it's effectively (per F-Line anyway) contingent on a decision to cut down the road anyway. Digging under Beacon is going to be a lot more expensive, technically harder, and considerably more disruptive (we know digging around those buildings can get dicey, fast, and they're constraining both sides of Beacon rather than on only one side on the Storrow side where there's more room to work). Between that fact and the fact that it really doesn't increase the walkshed by
that much, it's almost certainly going to lose badly in a cost-benefit analysis.