Back in 1996, Boeing came close to launching a 550-seat "747-600" derivative which featured a stretch of around 14m over the -400, a larger wing and was powered by either Rolls-Royce Trent or Engine Alliance GP7000 engines. The project foundered after it was unable to secure launch orders.
Driven by the MD-12 and MD-XX of the same era. Boeing shut down the MDC competition for the race to build the largest aircraft (a huge expence in an unkown market). Airbus took the challenge a decade later.