Well, apart from the small matter of West Station not existing (and who knows how many more rounds of Harvard back-and-forth it will take before something gets done with Beacon Park), it's not the worst idea in the world. (And if it catalyzed them to building West Station a bit faster, that'd be a good thing) Can't avoid at least a little tunneling at the Union Square branch junction to the GJ, because the GLX is on the wrong side of the ROW. I've always been a little wary of whether there's enough room to dig under the Fitchburg to get on-alignment from the GJ to the GLX incline to Lechmere, though F-Line among others says there is so I'm inclined to go with that part being possible at reasonable cost until proven otherwise. Without the B-line connection, though, GJ becomes a fairly longish northern branch of the GL, I'd start to worry a little bit about operations and on-time performance (though that could be mitigated if they did what they should have done in 2014-16 and opened up (and raised) the Brattle Loop platform at GC. (It'd be useful even with just the existing GLX, and they, uh, didn't quite do it right in the renovation...)