Saw this posted on the RST SWA Facebook page.
Southwest Airlines in Atlanta: No. 2 with no bullet
"But Kelly said he expects the opportunity in 20 years to be in longer-distance flying and bigger airplanes, “maybe some bigger, dual-aisle version of the 737.” He referenced Boeing’s new mid-sized plane under development, known as a 797, and said “maybe we’ll think about something like that one of these days.”
The same RST post had a comment from a WN pilot that listed an unnamed source (I know, I know) saying the day before the Paris Air Show that, "We are 5-10 years away from a bigger jet, but it'll have to fit our model. Maybe the 78, or something even newer." And then interestingly enough the 797 was announced the next day at Paris.