I expect to see Pac-Man and Miis as newcomers, though I've recently learned that Shulk's anniversary is actually June 10th which would be perfect for a reveal if he isn't unlockable, (and also they might actually have details about his new game at this year's E3 so it'd be perfect) so if we see him too, I also expect to see Palutena or Dixie Kong (whoever isn't unlockable) as I really think we'll see an even number of newcomers before release. Characters like Mewtwo and Ganondorf, I expect to see in the game, but as unlockables. And before people say we've already seen unlockables, I don't really think Marth and the others need to be unlocked this time.
As for veterans... I don't really think showing them at E3 is a good move strategically as they'll be easily overshadowed by newcomers - and before you bring up the direct, I think the direct was a great presentation for showing off all this info including veterans and a newcomer at the end. If they showed Greninja first we wouldn't have focused as much on the other info Sakurai had to get out there, like Sheik and Zamus being separate because of no more transforming characters, and the presentation wouldn't have ended in a bang. Whereas at E3, you have to start out with a bang to compete with every other company and trailer around grabbing your attention immediately with something not entirely expected. Therefore! E3 has to start with some newcomer trailers like last year, people were much more excited about newcomers like Megaman than veterans like Pit or Bowser. That's why at this point, after the initial announcement veterans at E3 seems like a bad idea, since they won't extend the hype or anything... They really get people talking at other times like the veterans on the site, when they have their own time to shine! Unfortunately, I really have no idea when else they're supposed to announce Ice Climbers so... maybe they'll be at E3 with some new mechanic or somethin.
Short version:
Newcomers - Pacman (fo sho) , Mii (probably) , Shulk (maybe) Palutena or Dixie Kong or Fire Emblem Awakening Rep being either Chrom or Robin (maybe)
Vets - Ice Climbers (I guess) , Meta Knight (maybe) , Wario (maybe)