The 747-400 has come very much to the limit of what the 747 wing can handle. An increased capacity 747 will require a new wing, revised landing gear and all such things - like the 747X proposal which was shelved about a year ago.
The 747X would have been an almost entirely new plane where little more than the 747 fuselage cross section would be reused. (Please do not confuse with the 747-400X which now has been renamed 747-400LR).
Have a look at the A380 wing. It has a 50 feet greater span. It is an entirely different sort of animal, and it has some growth potential for future stretched A380s. Just in case it should be viable to market such a thing in 2020 or 2030 or such.
Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs