That's going to be a tough cookie to crack. Even the small line improvements that have been suggested can't find enough traction to be budgeted, while there isn't even enough ongoing to repaint signage and keep up current stations. On top of that situations like the Ft Mac station that doesn't even have a current purpose.
IF these things are going to be done, more likely that it will come from some savvy real estate transformations on the valuable downtown station lots. And, even beyond that, Atlanta and surrounding counties are CAR counties. Some severe transformations would be required to even be able to house the vehicles for people that wanted to ride. For instance, the College Park station is generally out of spots all day until after five-ish. The added stations and infrascructure would require the purchase of a great deal of land to put the stations and lots, etc.
Given the lack of (past) planning and structure I seriously doubt anyone will pump that kind of money into a system that does as poorly financially as MARTA has traditionally.
Improvements would also have to be made in line structure and timeliness. A short ride 6 miles from Decatur to Little Five takes well over an hour and up to three bus transfers...

I would love to see a great deal of money and infrastructure continue in making Atlanta a true cycling city. If the Beltline people don't kill "themselves" with their ways, and the rest of the planned construction actually happens, that alone will be an invaluable resource to cycling Atlantans. I also hope to see the Comet and Panola Mountain systems tied in as one continuous ride. No matter what you thought of Mayor Reed, he WAS GOOD for cycling in Atlanta. I hope that tradition continues.