I fully believe the roster is going to get padded with Echoes this time around. Maybe "padded" isn't the correct word for a roster already pushing 70 characters, but I think what's going to happen is we only get a small handful of "true" newcomers, maybe even as few as three or four. That's simply an effect of bringing all the vets back.
But I don't think Sakurai will be content with that, and I see Echoes as his solution for adding in fan favorite characters there isn't time to add as fully fleshed options. Emphases fan favorite, because no, I doubt we're getting Funky Kong or Raichu just because they'd work quite well as Echoes.
Isabelle, Dark Samus and Shadow all fit nicely into the venn diagram of "popular character" and "echo potential". Characters like Waluigi, Takamaru or Midna might be popular enough to warrant inclusion as characters, but they don't have the echo potential. And characters like Raichu or Funky Kong may have the echo potential, but not really the popular backing.
Instead, characters like Dixie or Bandana Dee are more likely. Neither fits that well, but if the reason for putting Echoes in is to get more popular characters in, they both seem like good fits. If Diddy's truly lost his tail attacks, then technically nothing prevents Dixie from being skinned to his skeleton, even if the end result is a character that lacks Dixie's primary helicopter gimmick. Bandana Dee could easily be slapped onto Kirby's skeleton, maybe with a permanent B move as the only thing differentiating him (perhaps a variation on Kirby's Metaknight spin or Marth stab).
Impa has popularity within the fanbase and once again, her extreme ease of being implemented as an echo. Chrom, too, has a shot because of this even if I'd find the choice quite boring.