As someone who doesn't believe in the box theory, I think we're only getting 2-3 more unique newcomers tops. I've seen people suggest we have half a dozen more coming, even though we've only had five announced up until now. Every Smash game people have obscenely high standards and they're never met. Sakurai tried to let people down gently this time by telling them to "not expect too many new challengers" and people flipped and took it to be part of some long trick or something... developing a roster of over 70 characters is seriously no mean feat.
It's dangerous to expect more than what is reasonable, I feel. I understand being disappointed that one's favorite didn't get in, but I think people have to also have realistic expectations to some extent so they don't hype themselves up over nothing. Character-wise, the biggest priorities for this game were Inkling, Ridley, and K. Rool. Those characters were both popular in their own right and hailing from popular series. Those should be considered the mandatory characters and what people hoped and dreamed for - and we got them, and it's great!
It isn't like this game isn't already amazing or something. None of us could have dreamed of a roster like this during Brawl. But here's the simple truth:
Not everyone can get in.
If you were banking on a character to get in that wasn't those three (especially say, a random third party), and they don't make it... better luck next time.