Neither Airbus nor Boeing has ever produced a jet airliner which hasn't been either stretched or shrunk or both.
DC-8, DC-9, DC-10 including later MD
variants are no different. Same goes with Comet, Caravelle, 1-11, Tristar, BAe-146/ARJ, Fokker-70/-100. Etc.
There is no reason to assume that the A380 should stay as a unique exception produced in one variant only.
But like the B757 it may easily last many years until we see another variant.
Jet airliners produced in one length only:
Dornier 328 Jet
and several Soviet/Russian types.