As someone who has been to the last two D23 events in the last few years (and will be going to this upcoming one), maybe I can help out...
To start with, the last several big display in the main showroom have consistently been related to Disney parks in some way, shape or form because it was a major anniversary for them. The display of 2015 was all about Disneyland in general because it was the 60th anniversary. The display in 2017 was on Disney pirates, because it was the 50th anniversary of Pirates of the Caribbean. As such, this year will mark the 50th anniversary of the Haunted Mansion, so perhaps the big display will be on Disney spooks, scarier imagery and such.
It also helps to look at the anniversaries of Disney things to see if they will have panels devoted to them. I'm fairly certain there will be panels on "Sleeping Beauty" (that's having its 60th anniversary this year), "Mary Poppins" (55th anniversary) and "The Little Mermaid" (30th anniversary). It would also be great if they did a panel on Donald Duck, whose having his 85th birthday(!) this year.
As for new theme park attractions, I'm pretty sure they will probably announce all the new stuff pertaining to WDW's 50th anniversary, full details on the new Marvel land coming to the CA Adventure (with an official name), and new stuff in Disneyland's Fantasyland (or maybe even Tomorrowland).
In terms of the Disney Legends panel, I would love to see Tom Hanks (voice of Woody in "Toy Story", his appearance in "Splash" (the first Touchstone movie), and playing Walt Disney in "Saving Mr. Banks") and Frank Oz (voice of many a Muppet character (i.e., Miss Piggy, Fozzie, Animal, Sam the Eagle) and Yoda) inducted.
I'm sure most of this is going to happen, although it remains to be seen what they do for the Legends panel; Tom Hanks and Frank Oz are merely those I would like to see inducted.
