I'd like to see what they have planned for underground structures like mineshafts, strongholds, dungeons, etc. I speculate there's going to be layers to it, as there's going to be a "crust" layer that is virtually what we already have, including the underground structures. Beneath that would be where the larger open caves would generate, and obviously below that, there's the deep dark. There's obviously going to be modifications or sub-layers so that these layers mesh together well where they meet.
I'd been working on a fun idea for strongholds for about 2 years now, and while it's always been for the sake of fun, there's always been that hope that it at least inspired some sort of change to the shabby stronghold algorithm we have now.