Sorry about the huge size of the map but scrolling never hurt anyone (well, maybe).
This is a fantasy map, though most of it is based on either current plans or past studies.
Blue Line: This is the current North-South line extended to Windward Parkway. There would be five new stations on the northern end of this line.
Red Line: This is the current Northeast-South line extended into Gwinnett county. There are three new stations: International, Pleasentdale, and Presidential.
Other new stations on the red and blue lines:
Jackson Terminal - The new international terminal at the airport will have its own station.
Firefly & Whitehall - Both of these station are in old industrial areas ripe for redevelopment. The proximity to downtown, interstates and the main transit line would almost guarantee success.
Yellow Line: This line would be an extension of the current Proctor Creek and East/West lines. Six new stations would be built deep into currently underserved southern DeKalb county.
Purple Line: This line would also travel along the current East/West line. At the west end would be one new station, Fulton Industrial. The east end of the line would terminate that the Avondale station. Avondale is slated to be transformed in a TOD. This, along with a connection to the gray line would make this station relatively busy.
Black Line: Not currently on any long range plan or study, the black line would have four new mini-stations in the rapidly densifing Midtown and Downtown areas. The new underground station platforms would be small, servicing trains with only two or three cars. The purpose of the black line would be to reduce the time between trains, making use of trains more appealing to intown residents. MARTA was designed for 90 second headways. Currently the time between trains is as short at four minutes on the main line during rush hour and as long as twenty minutes on the northern sections during non-peak hours.
Green Line (light rail): This light rail line is most likely the first line to be built. It enjoys strong support from the neighborhoods along the path of the rail lines and is being championed by the president of the city council. This line would likely be built with private funds by developers along the route who see it as a way to open old industrial properties into high density mixed use.
Orange Line (light rail): When MARTA was originally conceived, a northwest line to Marietta was part of the plan. The residents of Cobb county opted to go with more roads instead of transit. Times have changed and Cobb is more open to a rail line. Besides the green line, this line has the second greatest chance of being built. The new 17th Street Bridge over the downtown connector includes a lane for the orange line.
White Line (light rail): One of the problems with the current rail system in Atlanta is that many residents of the northern suburbs have jobs that are located in another part of the gridlocked sprawl from where they live. The white line would help move riders across the northern part of the metro, connecting the various employment centers together.
Brown Line (light rail): The brown line would help connect south and west DeKalb county residents to the rest of the transit network. It would also service Turner Field.
Gray Line (light rail): These has been strong opposition to rail lines by neighborhoods along the gray line. For this reason, I've left the stations off this line since the route would have to be carefully designed to prevent opposition. Emory and the CDC have been very supportive of rail proposals and would certainly have at least one station on this line.