I certainly hope they don't. In fact, the amount of characters we know on the roster now is already stretching the edge my limits. A big part of why Smash is so entertaining to me isn't the competitive side of it, but the massive amounts of content, collectibles, and in particularly, unlockables. It leaves a sense of wonder and surprise about which what you don't know-- the whos and whats left blank for you fill them in. That's always been more fun than the competitive or social part of the game anyway, at least for me. When you experience a game for the first time, that's when it's at it's peak of excitement, I would hate to lose that months before the game is even released.
If all the characters are leaked ahead of time... I'm not sure how I would react. I don't want to believe I'd be so dramatic as to boycott it's release, however I can't see myself enjoying it nearly as much if that were to be the case. Telling who the characters are ahead of time removes the most valuable unknowns and crushes any sense of fulfillment, accomplishment, and surprise. And I suspect Sakurai aware of this-- after all, there were secrets on the Dojo website that were optional to see while looking up information on the game, I doubt he'll tell us everything about the game beforehand.
tl;dr version: I would expect a few more characters, perhaps 5, to be presented at E3. Leave another group, 10-20 depending on the widely variable roster predictions, to be complete unknowns until the game is released.