In my view the 747-400 is getting close to as big as it can go, problem being landing gear.
777-300ER is running into problems with the high PCN/ACN required, very few runways in Australia can actually handle such weights, look at airports that have high volumes of 777 traffic you will see you they are actually moving the flexible pavement.
Over time you can see spots where the 777 brakes, the pavement under the aircraft actually moves with the aircraft causing large bumps in taxiways.
A 747 advanced will have a high PCN/ACN with the higher weight still distributed over 18 wheels, without a gear redesign to have the weight over more wheels this aircraft could be limited out of more airports than the 777-300ER or A380.
