Boeing will respond at some point after the 787 gets going with a larger 747 in a similar way that they did with the 737-800 (Really a new airplane flying under the type certificate of the existing airframe... Which Airbus REALLY hates.).
I’d say in 5-7 years the 747x (650-750 passengers) will resurface as more airport infrastructure is built to handle the larger aircraft.
After that, Boeing will have the kinks worked out of their cargo variant of a blended wing-body aircraft. Say in 10-15 years expect an aircraft that can haul 1,000 people, yet able to fit into the infrastructure of a 747...
Bet that'll be a comfortable ride...NOT!
I already insist that only people weighing less than 110 pounds can be truly comfortable in coach class as it is.