Well the trouble is really polymorphism, and I was actually thinking more about polymorphic data structure: i.e. data structures where the elements are required

Well the trouble is really polymorphism, and I was actually thinking more about polymorphic data structure: i.e. data structures where the elements are required to have certain methods callable on them. I agree this is typical OO design and could be approached in other ways, but none I know of are satisfactory.