It's not so much as there is a "longer version" designed and sitting around, but more that the wings were built with a potential future, longer fuselage variant in mind. This is more evident if you look at a top-down view of it and note that the wing-to-fuselage ratio is higher than that of other planes, similar to how the 747-SP looks much shorter in length than its wings should be designed for. There's more details on the Wikipedia page for it: https://en.wikipedia.org/wiki/Airbus_A380#Variants_proposed_...