Let us consider hypothetically stretching the B747-8 another 7 frames to a length of 79.83 meters.
Let's lengthen the landing gear by, say, 8 to 10 inches in order to avoid tailstrikes and to allow clearance for GEnx engines with the 111 inch fan.
OEW should be roughly 460,000 lbs. Nominal seating would be about 500.
Based on the weight difference, range with a full passenger load would decrease from the 8000nm of the B747-8 to about 7300nm. However, the lower SFC of the larger GEnx engines would reduce fuel burn by perhaps 4%. Range might be close to 7700nm.
Total development and certification cost should be below $1B. Is there a market for such a large aircraft that would justify spending $1B on development?
Obviously, taxiing would be a problem at some airports. Some airlines would want greater range, though it still exceeds the range of the B747-400ER.
What am I missing?