I had a shower thought earlier today. There are three characters that are very likely to be newcomers or echoes. They are: Shadow, Skull Kid, and Isabelle.
Shadow has not yet been shown off as an AT. His AT may have been replaced by Knuckles. A new item called Timer functions pretty much the same way as Shadow's AT did.
Skull Kid has not yet been shown off as an AT. His AT may have been replaced by The Moon. A new item called Ramblin' Evil Mushroom functions similarly to Skull Kid's AT did.
Isabelle has not yet been shown off as an AT. Her AT may have been replaced by either Kapp'n (the new Animal Crossing AT) or Chef Kawasaki. Chef Kawasaki functions similar to Isabelle's AT did, by giving out food to the player (after using Kirby's Final Smash, of course).
Based on this info, I say that these are the big three.
Shadow has, I'd say, a 90% chance of being playable.
Skull Kid has, I'd say, a 93% chance of being playable.
Isabelle has, I'd say, a 94% chance of being playable.
As for Isaac and Geno, I'd give them about a 70% of being playable, since they're both very popular picks, and the tweet Sakurai made that may be a hint for Isaac, but may also be a coincidence. I dunno. However, not only did Isaac's AT not appear in Sm4sh, but also I heard that recently his music tracks were removed from Metroid stages, which could mean that Sakurai is doing away with Isaac, or that he may be getting his own stage. Either way, it's looking fairly good for these two.
As for Dixie Kong, and Impa, it could go either way for any of them. I'd give them about a 50%, since we haven't seen any clues or hints that could lead to them being playable. Plus, if we get Skull Kid, that means we'd have a new rep for both Zelda and DK, and that may hurt their chances a bit.
Obviously, popularity seems to be what Sakurai is going for, and these characters are all popular picks, especially Isaac and Geno. But I'm sure there are other popular choices I missed or didn't think of. But I say if we're gonna get anyone else, it's gonna be Skull Kid, Shadow, and Isabelle.