This sounds promising, but I've always thought of a solution that is within reach of 10-20 years if it starts now. Simply extend MARTA service to include all of the large density counties. It's an Idea that was thinking about years ago, but really can happen now.
It's do-able, because all that MARTA would have to start with is merging with the existing lines outside of MARTA's Service. For Example, Merging all of CCT's lines, with MARTA, same for Gwinnett's transit system, as well as Clayton county's. (They do have one right?). MARTA already takes care of Fulton and DeKalb Counties, with enough lines, so that that's 2 counties out of the way. Once the merging with the outside lines is complete, the extend lines into Douglas, Henry, Rockdale, Forsyth, and Cherokee Counties.
Once that is done, Divide the service area into 5 zones: NW, NE, SW, SE Metro lines, then A or something like it as Atlanta City lines. Each zone will have it's own comand posts like NYC. Then later as counties outside of the new zones develop, population increases, more lines can be added, and counties to be included.
It's seems like a lot, but it can't be any more expensive than every county running their own system, and purchasing their own buses, and such. It seems like it could be done.