With a small roster, and small aditions over time, keeping balance will be easier.
If they start well, the 16 characters we will have at launch will be viable and good, or something close to that, and then, they release one character, and they work the next patches until everything gets balanced.
I mean, Mobas follow that model, and they are unbalanced by nature, but i think mobas have much more variables that makes balancing harder (we release this character, how he will fit with the items, and others characters, etc), but when you do a Fighting game character, you need to think on his matchups (which is still a big thing, but small aditions to the roster will not change the game that much).
I think they should focus on the characters being fun, and they are showing that.
Maybe that will be the end of joke characters, at least from the balance point.