I'm told that wing redesigns (edit: for tapered, swept wings) are difficult, so variants of a model often do with a common wing optimized for the middle member.
But then I read the following:
Based on Antonov's earlier An-124, the An-225 has fuselage barrel extensions added fore and aft of the wings. The wings also received root extensions to increase span.
That doesn't look so difficult. Of course, the wing root would be a new design, but we still get to keep the rest of the wing. Perhaps the flutter modes are less favorable......but wouldn't that be more or less inherent to higher spans?
So: Is/why isn't this method used to design variants or derivatives of large airliners?