Gotta join in voting for Prof Tolkien. Not for amazing literary qualities, but because his is the pen that launched a thousands books. Talk about sea changes in litterature in the twentieth century, writers who redefined our relation with fiction, and his name is right up there atop the list.
Orwell is also a strong candidate.
The Nobel committee is frequently on the wrong side of history, but they did give Tolkein a rather damning review when he came up for consideration. That said, 'classic' or 'canonical' status is usually a matter of influence rather than quality, so I suspect he will, in time, be given the nod.
A fair few 20th century works are already considered classic, I think - Steinbeck, Orwell, Hemingway, Verne and Golding have been mentioned, and I would add at least Joyce, Woolf and Conrad to that list. As for authors not yet considered classic who might in the future, that is an interesting question. Asimov perhaps, but I can't see science fiction being considered along with 'high literature', at least for a while: Verne still suffers from that. I'll throw in Gabriel Garcia Marquez and Umberto Eco as my bets.