Would an increase in length, rather that raising the floor to get in more pax have better economics? Viewed alongside a 747-400, the current A380 models look rather short, with an aspect ratio more like a 747SP.
Fuselage length against wing span does not mean anything. The wing span/area is a requirement of the aircraft's mass - not length. The A380 has a higher mass than the 747.
However, Airbus do have tentative plans to stretch the A380-800 to an A380-900 (but not yet).