Clearly the biggest balance issue will be the fact that I'll be playing on-line which will naturally cause everyone playing me to lose, which will lead to calls of imbalance, because I'm just too good.
Wahahah.... yeah so,
I'm gonna have to say that Link and Mario will be two of the strongest characters still this time around. I never thought Mario was that great in Melee, until I realized that he actually can dominate completely, now none of my friends can beat me as Mario, so I'd definitely say that there are many hidden levels of kick-assness in many of the characters which can take some time to figure out.
Also, Nintendo is likely to give surprise strength advanrages to characters that you might not expect it from. For example, I'd say look for some surprise domination from beefed up characters like possibly YOSHI, PIKACHU, and possibly Pokemon Trainer. Certainly, the characters which are notoriously kinda lame (like Peach) will be retooled a bit so they're not seen as the "weak" characters yet AGAIN. So my bet is that the usual "underdogs" or less-popular characters will suddenly be real contenders.
Of course, like I said, I will provide serious imbalance by destroying you all, and it will be done with Link. Beware!