I suspect the Direct will be focused primarily on game features - modes, changes to structure, that customization thing for the 3DS version, things like that. We haven't heard really anything about that, so a Direct would be a perfect time to make the skeptics have a reason to buy the new Smash outside of just new characters.
It'll definitely have at least one newcomer, and I'd be surprised if we didn't also get a few veterans announced. Yoshi, Wario or even Snake would be good guesses. Newcomers-wise, I think they might decide to open with a newcomer and close with a newcomer to keep hype up - good openers would be characters like Mii or Palutena, who are exciting but not incredibly hyped, and closers should be either a new third party character (Pac-Man, Bayonetta?), a highly-hyped and demanded Nintendo character (Ridley, Mewtwo) or something that's a bit out of left field and would be unexpected to most (Wonder Red, Takamaru, someone no one will see coming...).
I'm definitely with those that think we'll see a release date, too, though I still think it'll be Holiday 2014. They might give a month instead of a specific date - maybe November 2014?
Either way, I'm sure the Direct will be a blast.