There might be interest in a 15" Air, but I don't believe it's in the cards.
The retina models have accomplished many of the same things the Air line was created for. A 15-inch "Air" form factor is unnecessary. I fully expect that we will have a 12" Air with Retina sometime next year and the wedge shape won't come with it. The 13" MBP will disappear (maybe sticking around for edu only). Then, once it's cost effective, the Air as we know it will be dropped, and we will only have the Retina line that keeps the same form factor as it currently has.