Okay, if we are going to get another character revealed tomorrow--and mind you, that's a big if--then chances are, it'd be a returning veteran. Nothing more. Here's a list of characters that are most likely going to get confirmed:
Mr. Game & Watch: Given the fact that he was "leaked," and Sakurai revealed over on twitter that he isn't quite ready yet to be added to the roster, this is pretty much his only chance at being officially confirmed before Japanese launch.
Falco: Setting aside the Petilil leak, tomorrow might be his big chance at returning all while giving Wolf a good sporting chance (and who knows? We might even get a new stage out of it). He's a fan favorite, and if my theory regarding version exclusive starting characters does turn out to be true, we could very well get him tomorrow.
Meta Knight: Aside from Captain Falcon (who's likely to be an unlockable character again anyways), this guy's pretty much the biggest name in Smash to reveal, and even in Japan, his confirmation would be a big deal. I doubt it, but we'll see.
Wario: See Meta Knight above, only a bit more probable.
Ice Climbers: Before I even learned about this, I always predicted these two to be the next returning veteran to be announced, and so my assessment remains, even if the circumstances revolving this event may contradict said assessment. If we don't get anybody tomorrow, it's going to be a locked selection.
Ness: A fairly distant possibility, but could happen if Lucas is to return.
Chorus Kids: Considering that this is a Japanese event, wouldn't the best shot for another newcomer reveal be these guys? You know, big in Japan; not so much everywhere else? Granted, if they don't show up, I'm going to predict a post-Japanese launch reveal alongside Jigglypuff, G&W, and ROB. But of all the newcomers, this just makes the most sense. Not to mention that teases and leaks like the Sneaky Spirit are often used to fuel speculation regarding newcomers.
Shulk: A consideration due to the Mr. E leak and nothing more. It could happen; but if nothing else, if he is indeed going to join the fight, I'm getting the vibe that he'd just be a Secret Character--as in, an unlockable designed to be one of the last--if not THE last one to obtain.
Takamaru: While I do not believe for an instant that he's going to be playable, I'll keep an open mind for tomorrow just because he is to Japan what Little Mac was to America.
Beyond that, there are no other newcomer candidates that are going to be confirmed. Nor returning veterans, though fellows like Ganondorf and R.O.B. are far more likely to be announced than Ridley, Chrom, Isaac, or King K. Rool among others, no matter what the demand for them may be.
As far as who I think will be announced, I'll say Falco is the most likely candidate for confirmation if we are to add another character to the roster tomorrow. Otherwise, most likely character that we are going to get next will be Ice Climbers. Most likely newcomers? In August, it will be Ridley.