The brand best positioned to take Coca Cola's place in an ATL would have to be Moxie, nowadays a regional brand of New England where it was invented it was once much more popular across the country and it technically predates Coke, so it's pretty easy to come up with a way that Moxie dominates the market instead and continues to spread worldwide. It even had presidential endorsement in the 30's (the preferred drink of Calvin Coolidge). The only big impediment AFAIK is the taste, it is extremely bitter.
My state used to have a lot of soda manufactureres - for regional consumption, of course. These days, the last remaining one is Yacht Club, which is actually really good. Maybe if Yacht Club hooked up with Moxie to improve the taste, could that help?