This is a great question. It really is so hard to predict what will become a classic.
I think your additional thoughts are very useful ones, @JenniferD: for the purposes of this forum I've suggested a classic is any text by a dead author. But I suppose what we're really talking about here is the 'canon' - books by dead authors that actually continue to be read and valued. Of course every literary critic under the sun has had an opinion on what texts are canonical and which aren't, and those opinions have been very influential: many of the classic authors we hold dear today we only even know the names of because some university professor decided everyone should know about them! Think Harold Bloom for e.g.
I think, or at least I hope, that going forward works from authors of different cultures, races etc. will be considered classics much more. The canon at the moment comprises mostly of dead male Europeans - which isn't to say that a lot of their work doesn't speak well for the entirety of human experience - Shakespeare, of course, is applicable in just about every culture and situation because he was so good at distilling universal human characteristics. But I think most people would agree with #WeNeedDiverseBooks.
My best guess at what will become canonical texts in the future: 1) books that represent the experiences of diverse ethnic and social groups. 2) Books that appear frequently on internet lists! This sounds like a silly one, but I do think social media will have an impact on what books we value in the centuries to come. More than ever before we know what everyone else is reading, and I think the books that continue to stay on "Best Books of the 21st century" style lists will be the ones we will continue to value 100 years from now. Book bloggers may just replace professors in influencing what we continue to read -- perhaps we Rifflers are shaping the future canon!!!