Atlantic only occasionally will be fairly crowded on a special convention event or private event, but much of the time, it seems to have just handfuls of people there. Jelly Rolls on the other hand almost always has a good crowd. I would think if Disney is smart they'd keep a new concept somewhat similar to Jelly Roll's, just owned by them instead of a third party. 'Jazz' doesn't really say much, as it can range from Dixieland, big band, light rock, to bebop, freeform, etc.
Atlantic is a giant space for such light use - and really would be an awesome restaurant space - would love to see a great mid-priced steak/seafood/American/mix type restaurant go in there - utilize the outdoor patio/balcony for waterside seating, lovely two story views over the lagoon with the boats cruising by...then they could do something else with the Big River space and wouldn't have to worry about fryers/ventilation issues with the rooms above.