May be ridicules - but imagine a B-742 re-equipped with a pair of triple seven motors. A cheap efficient workhorse alla the old 70 series stretch 8 (and with plenty of power to spare).
Boeing sells new airplanes however, and would simply buy up and destroy classic airframes if the idea worked. I had measured the pylon / ground clearance of a B-742 a while ago, and a 777 motor would clear - although a tight fit (at 20 inches to ground - 4 inches less than a -70a).