The English Fact Sheet reviewing the alternatives is here:
https://cdn.mbta.com/sites/default/files/2022-09/2022-09-26-english-alternatives-analysis-v_2.pdf
There are some interesting ideas in here. I remain astounded that alternatives featuring extended mixed-traffic-running remain under consideration.
To be quite honest, I believe that is disingenuous for a public presentation like this: SL3 does not run in mixed traffic, charges a rapid transit fare (I believe), and is branded as rapid transit; mixed-traffic-running is antithetical to all of that. It would not be unreasonable for a member of the riding public to hear that SL3 is being extended, see that an extension to Everett is proposed, and conclude that the experience through Everett would be simliar to the current ride through Chelsea. The public should not be asked to do a cost/benefit analysis of the fine print of a proposal like this; they should be able to look at a map and be able to say, "Yes, that would help me go where I need to go," or "No, that doesn't go where I need to go."
(I suspect many riders are indeed savvy enough at this point to recognize the implications of mixed-traffic-running, but again I believe that it should not be encumbent on the public to have a honed BS meter on these topics.)
That said, most alternatives require minimal mixed-traffic-running: Alts 3, 4, 5, 6 and 7:
I think all of these proposals have merit. I think I like Alternatives 3 and 7 the best: extend the busways to Sullivan Square (and maybe beyond), and either through-run SL3 buses all the way, or add on a layer-on service to increase frequencies within Chelsea. (Though I do wonder whether this analysis is rolling-stock-neutral, or otherwise accounts for increased rolling stock needs.)
We can get double-bang-for-buck with Alts 3 & 7: those bus lanes on Lower Broadway and the Alford Bridge can be additionally used by the 104 and 109 (which hopefully will also get bus lanes extended all the way up Broadway in addition, separate from the SLX project). With those speed and reliability improvements, one or both of the 104/109 could be extended south of Sullivan, to Kendall, downtown, or both. That would cover most of the benefits offered by Alts 4, 5, and 6.
(Obligatory plug: if we keep building BRT, we should start differentiating services; a new radial service to Everett -- such as an improved 104/109 with extension south of Sullivan -- could be given a new identity,
as I describe at great length here.)