The busway can be dual mode with track embedded in the floor, supporting both trolleys and buses. Buses for Logan and Chelsea; Trolleys to Design Center (maybe eventually out into Southie if they ever get over their transit aversion.)
Thought has always been to connect the South Station end of the busway back to the Green Line at Boylston in some fashion. Big expense regardless the routing, but a doable, important connection. Then you have Red, Orange and other Green Lines connected out to the Seaport.