After a lot of analysis and such, I am still sure that Ridley has a big chance to be playable. But I won't go too in-depth, since I suck at explaining stuff.
But, one thing I did notice, was that, you know how Charizard is so new and changed now, that they even gave him a Newcomer-like Introduction in Greninja's trailer, right? Of course, he's not a newcomer, so it seems they did that to indicate that Charizard is very changed (no Trainer, and very different moveset). So he got a debut in Another newcomer's trailer.
But, Sheik and ZSS also got a Newcomer-like Flashy "Zeroes In!" and such. Since they both are so different. So, it would make sense for them to, just like Charizard, be "introduced" in a Newcomer's trailer. And what better trailer to introduce ZSS, than in a trailer about Ridley?
Sheik could do the same thing, as I personally expect a new Zelda character (Tingle or Ghirahim).
I realise they did this with Yoshi as well (Rolls in, I Think?), and it's very unlikely there will be Another Yoshi character. But I Think Yoshi got that since, as Sakurai said, Yoshi is very different now.