Were I to guess what we would see at E3, I'd say:
Veterans:
Wario
C. Falcon
Ness/Lucas
Ice Climbers
A 5th veteran.
Newcomers:
Pac-Man
Ridley
Mewtwo(Not technically a newcomer, but given the newcomer treatment.)
No Palutena?
Also, here's what I would like in E3:
Instead of having demos on the Eshop or the show floor, have small stands where you could play 1-on-1 matches with a friend, one of them playing as a newcomer, to show people the uniqueness of most of the newcomer characters, such as seeing Villager or Rosalina play.
In the reveals, for veterans they should show Wario (maybe give him a new side-B, showcase that), Ice Climbers (to show off duos, and possibly nerf them), and Meta Knight (show what replaces his gliding).
Newcomers should be at least 3 like last time, but throughout the whole Nintendo live stream during E3, show them all at different times. For example, maybe they can detail Tomadachi Life a bit, then show Mii. Introduce Palutena along with the basic Smash Bros news, and maybe instead of a third newcomer they could talk a little bit more in-depth on Greninja.