Perhaps if when we go to 1.7.2, if there was a fee, per se to have your stuff copied over to the new world? That way players who wanted to keep stuff were capable. The only things that wouldn't copy are entities. So technically if you really just wanted everything you've collected and not so much of what you built, you could make a stack of chests filled with your items. Now of course The fee would probably be real money, or perhaps even ingame based off how much room it takes. Just a quick thought I had is all ^^