Gwinnett will vote on whether or not to join MARTA in March. No commuter rail extension is planned, but a short heavy rail extension is.
Part of Cobb is likely to eventually vote on joining, too. What they would build is not clear yet, but while Heavy rail is unlikely, commuter rail is a distinct possibility.
Clayton County has already joined MARTA and they are actively planning a commuter rail line to Lovejoy, and they have money to build it. Problem is, the line ends at East Point. This allows for a transfer to MARTA heavy rail, but East Point is not much of a destination itself.
So the prospects for commuter rail in Atlanta are better than they've ever been. While the 2013 plan for Downtown Atlanta was so pie-in-the-sky that it never had a chance, something right-sized and in the right place would clearly be a good thing.
On the other hand, read this article:
http://www.threadatl.org/2018/01/23/gul ... n-atlanta/
CIM Group, the developer for the Gulch, has planned a parking deck that will take up literally all the ground level space in the Gulch, leaving no space for tracks and platforms. Whether this is a serious proposal, or a strawman to get officials to cough up incentives, I can't say. Much of the negotiation for this is going on behind closed doors, so, who's to say.