The plumber, who is actually the homeowners relative, wants to run a separate 3" waste line up there and tie into the main stack (in the picture) from the crawlspace. He said code where we are prohibits wet venting, although it's fine across the river, about 10 minutes from us. He'll run a 2" waste line off the new 3". The joists are only 2x6, so we are hopefully going to leave all the fixtures between the same 2 joists. Otherwise, we will have to drop a hallway ceiling several inches and go underneath them. It seems simple enough that it's worth doing, and putting a master suite in the attic will be a fun long term project.