A lot of the 'classic' literature that's still being read is actually what would have been popular culture, rather than anything particularly highbrow - Shakespeare, Pepys, Conan Doyle, Dickens, Collins, Stevenson, Mary Shelley, Stoker, Wells - basically, what you need is a selection from murders/crime, affairs, monsters, animals, magic and science fiction to ensure longevity. And, of course, the Hero/Antihero quest is essential in this.
So look to movies. If something has been turned into a popular movie or several movies rather than at most a single, worthy depiction or has become a template for others/being referenced in cartoons, other movies or images, the chances are that this is something people will still remember.
This may disappoint some to think that the authors who will still be in print will be largely the entertainers, but it's human nature - and as literature is all about encapsulating human experience and nature, it makes perfect sense to me that in addition to the writers above, it'll be Adams, Tolkien, Christie, King, Herbert, Puzo and others with more universal appeal/ideas that translated into moving images who have become part of our cultural legacy.