I think Birmingham, Alabama has to be one of the southern cities mentioned. Earlier in WC Qualifying, a game was played there for the US National Team and it seems like there would be enough support to play there.
It seems smaller states like Alabama and South Carolina are more soccer inclined making Charleston or Birmingham easier to manage. I think Atlanta wouldn't be very successful because it would be overshadowed by the Falcons, Braves, or maybe the Hawks(they are pretty bad).
If a team is to go into the south, a key to being successful is being the major team of the city.