Sonic Central predictions tho, for real:
Official announcements for Sonic Colors Ultimate and Sonic Collection.
Sega HARDlight has indicated they'll have announcements in there so possibly some new Sonic apps/additions to Sonic Dash or Sonic Forces Speed Battle or whatever.
Maybe a look at Sonic Prime?
WAY too early for a trailer for the second movie, so probably not that, but perhaps some kind of update on that. Just a brief "Hey, this is a thing" kinda thing.
MAYBE a look at a new 2D game with a smidge of a peek at a new 3D one (that might not come until early next year)
Some new merchandise for the Sega Shop website. Toaster or something, I dunno insert memes here.
That's kind of what I'm thinking, I don't know if they'd realistically have anything else. I imagine NINTENDO is in charge of Smash reveals, so even if it is Sonic related, it'll happen at E3 regardless. The only exception I can MAYBE think of is if there is indeed another Mario/Sonic Olympics title and even then I wouldn't be surprised if Nintendo announced that themselves.
I don't expect it to be very long.