Not right on the Midtown Mile, but Flywheel will be opening a new location in the Regions Building at West Peachtree and 14th. It was announced a few months back, so it should be open pretty soon. It's a great, high-end spin studio--I'm a big fan myself. There are around 25 of them in the US, and currently the only Atlanta location is in Buckhead.
This will be a great amenity for Midtown and draw a lot of people. The one in Buckhead probably has 500+ people walk through its doors every day--far more than a traditional retail outlet.
Y'all should try it out once they open.
Fixing the sidewalks one West Peachtree between 14th and 10th would be opportune, btw. With AMLI and Skyhouse (which is finally getting a restaurant, if I recall), there's only going to be more and more foot traffic on that street and it's currently just atrocious.