The Amateur Planner proposes a paved transit way with tracks for not just the B Line but the 57 and BU bus routes.
While we've never done this, other cities have, and it would have a whole host of advantages, including getting buses out of traffic (and bike lanes), stations better able to serve people with disabilities, even some additional parking spaces. Throw in the long-fabled transit priority for traffic signals and you might even see shorter commute times.