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Several years ago, Boeing actually had plans for a stretch version of the 747 that would have seated well over 500 passengers and competed with the Airbus A380. Airlines did not show any interest, so the project was dropped. The plan was to add plugs to the main fuselage that would have extended the length by around 35 feet, IIRC. The "hump" on top was to remain about the same length.

Back in the 1980s, McDonnell-Douglas had plans for a double decker (the "MD-12") that would have seated around 600 passengers. Again, there was not much interest in the plane. Based on the drawings, it was a far better looking aircraft than Airbus's whale jet.

The verdict is still out whether the A380 will even break even (especially considering the huge discounts reportedly given to the early customers).