Forgotten in history is the 757 languished for a long time with white tails lining the ramp in Seattle. It took quite a while before orders started coming in.

The 727-300 never made it beyond paper and there were various efforts including two concepts which did away with the FE. One incorporated many ideas including automatic reconfiguring of air and bleed for engine starts and air conditioning packs, an electrical heirachy like the DC-9/MD-80 along with other improvements. The other essentially moved switches and lights to the front just increasing workload. And neither moved beyond engineering mockups.