Is AirTran only looking at new aircraft (AA/TW 717 excepted)? I heard a rumor a couple months ago that AirTran offered to buy USAirways' entire Super 80 fleet but US insisted that the pilots be included in the deal.
I keep hearing that 757s are likely but that seems to be a pretty large jump, capacity-wise. 737NG makes more sense to me.
IMHO MD90s would make an ideal fit. I imagine there is a great deal of commonality with 717s. Does the MD90 have ATL-west coast range? Delta's fifteen could make a great, relatively new, addition. I don't understand why DL would want to keep such a small fleet around in these times. Also, I'm sure the JAL/JAS merger makes them redundant there as well.