I think it'll depend on how many characters they want to keep secret for the game. Most of Brawls unlockables were veterans, so most of the surprises were revealed before game release, a fact many people have criticized. While I think some of the Newcomers may be unlockable (Greninja doesn't seem like a starter to me), I'm hoping Sakurai won't tip his hand on all the Newcomers.
That said, assuming they haven't been revealed before then, some of the probable starters like Wario, Ice Climbers, and Metaknight would be obvious reveals to include. Then, I'd expect 2-3 newcomers, a highly requested character (Mewtwo, K.Rool, Palutena, etc), Mii, and possibly a Third Party. While we saw a Mii in the Find Mii stage, I don't think they should be counted out as Miis were everywhere in Mario Kart Wii, but you could still race as them.
I'm also expecting their to be at a minimum a press demo, possibly more with the game coming out in Summer for 3DS.