A third character is not impossible, but I think it's fairly improbable. Revealing both Ryu and Roy is a megaton as far as Nintendo would be concerned, if they assume we didn't see the leak... Which they kind of do assume that, do they not?
You can't really claim to know what Nintendo is thinking. They might consider it a megaton or they might not consider it a megaton. Your own opinion on what seems like a megaton doesn't mean anything.
During the last two years at E3 they have announced three characters each time. So however you look at it, two would be a step down.
You could argue that would be because it's dlc but I don't consider that to be a very compelling argument. There's no way to convincingly argue it one way or the other. It being dlc might mean the situation is different or it could make no difference to Nintendo (and Sakurai) in terms of how much content they want to reveal.
I'm not saying they'll necessarily have trailers and gameplay footage for all of the characters, they might pull another Mewtwo type reveal.
Sorry, my bad!
I automatically assumed in my minds eye it would be small, mainly because it's about things we know about...
And that's the reason I said "surprise", because it probably won't be!
Actually I just saw this post on reddit:
https://www.reddit.com/r/smashbros/comments/3914u9/new_sakurai_post_0608_the_update_that_includes/
"The update that includes Lucas will be available beginning 08:00 AM on June 14 (PT), 17:00 (CEST) on June 14th, and 01:00 AM on June 15 (AEST).
We made a special video broadcast to talk about the update. In it, I'll be on camera to give you all the details!
We'll air the video online right before the update, at 07:40 AM on June 14 (PT), 16:40 (CEST) on June 14th, and 0:40 AM on June 15 (AEST). Please check it out before the update becomes available!"
So it seems the direct will last around 20 minutes. I still think there's bound to be a "surprise" in there somewhere though.