What about modern classics? Pick a book published since 2000 and say why you think it will be considered as a “classic” in the future.
One title instantly popped up in my head reading this question: Harry Potter! Although the first of the series (Harry Potter and the Sorcerer’s Stone) was first published in 1997, the last one was only released in 2007, so it could be considered a product of 21st century. I considered Harry Potter as future classic not because of the books’ enormous number of sales, and not only because it set a new trend in literary world, but firstly because there are so many hidden germs of philosophy inside the fictional wizarding world created by J.K. Rowling.
Perhaps one day, Harry Potter will have its own section in literary study-guide sites like Sparknotes. You would probably find summary, character analysis, themes-motif-symbol study for Harry Potter, just as you’d find ones for The Great Gatsby today, for instance. The more I read Harry Potter, the more I believe it’s not just a fantasy-adventure tales. Inside there are lessons about our relationship with God, about good and evil, light and dark, love and hate. Oh, there is even much more symbol and metaphor inside Harry Potter than in The Great Gatsby.
Another symbol is within Harry Potter’s relation with his mother and Voldemort. When Voldemort cursed baby Harry with the deathly curse, he failed at killing him because of Lily Potter’s love which was planted inside Harry’s soul. However, Harry is also connected to Lord Voldemort, that they can read each other’s mind. I think it’s the symbol of good and evil in human’s soul. Dumbledore insists that Harry must try harder to close his mind from Voldemort, just as we must close our minds from things that provoke us to sin. So on one side, man has a small part of God—the Love; but on another side, he also has tendency to sin. But on top of that, love will always win against evil. How lovely are these symbols!
And all that makes Harry Potter deserves to be considered classic, even now.