I think there will be an A380 again, unless you think air traffic will go down, and I don't see any trend of that. Number of runways is a much tougher constraint to expand than plane size, so the segment of point to point occupied by A330/350 and B787/777 will ultimately be also saturated and require bigger planes.
Those runways still serve a ton of 737/A320 flights that have plenty of room to be upgauged to widebodies, and most of those runways could increase capacity further still if FAA/ICAO decide newer tech can safely reduce IFR separation.
Even with runway congestion, a big problem with an A380 successor is that any new commercial jet needs the fuel economy of two engines to have a chance of selling enough to be worth developing. Which in turn means that an A380-sized twin engine plane needs engines with ~50% more thrust than the current world record from a GE9X, which is already approaching material limits...
A new 747-like plane with a partial double deck might eventually make business sense to develop within our lifetimes... But I think a blended wing (lol) is more likely than ever seeing another full double decker like the A380.
But intuitively efficiency per passenger should only be the binding metric if you are not capacity constrained. ie it may be economical to fly four-engine planes with nearly twice the number of passengers, than not meeting the demand on a route because it is saturated.
