As L.1011 correctly stated, a heavier 747 will not only need a new fuselage, but also a new wing. The 744 is really stretched to the limit of the current wing.
When taking off with a heavy load the 744 cannot stick to the 250 kts. speed limit below FL100 without flaps extended. Some upgraded engines won't do the job.
It's no big problem on the 744. But anyway flaps means both drag and noise. Drag means also higher power needed for the same climb profile, and therefore even more noise. (And of course increased fuel burn).
Still no big problem, but on a significantly heavier 747 the problem would escalate.
In that respect the 380 is in a different class since it has a 50 feet wider wing span.
The bump, area ruling and the last 0.01 or 0.02 Mach number will not be the deciding factors for future large airliners.
Any future large ship will at least have to match the noise properties of the 380. Otherwise they will have to pay a significant noise penalty at many airports, or they will be banned altogether.
Revised fuselage, new wings, larger engines, tail to fit, you see, there isn't much left.
It is common practice to regard the 747 and the 380 as sort of competitors, where the former is just a little smaller. And efficiency factors are compared down to fractions of cent per seat/mile. But in the long run the 380 may be the winner simply because noise rules make it the only realistic choice for the airline companies.
And in the other end of the size scale the 777 will the same way outperform the good old 747.
Always keep your number of landings equal to your number of take-offs