Well, they're not supposed to be preview books anymore, so I don't expect a heck of a lot of solid information about the current iteration of the rules. Rather, I expect we'll see a lot of "in the first version of the fighter, he could do X, Y, and Z. We threw out X because it was easy to break, and found that Y had some bad synergies with other feats. Z is still around, but you might not recognize it now..."
There are probably a handful of key "reveals" that are planned for the book, but I think it's probably only about 50/50 that we'll see the "official" list of classes and races. I expect a print documentary about developing the classes and races, with a partially complete list at least.