I think the difference between clones and echos is that echos fundamentally play like the original, hence why, for example Lucina and Marth are so close o the tier list, but Dr. Mario and Mario or Fox and Falco are so far apart. Then there's the fact that echos seem to have the same proportions as the original. So it doesn't make sense imo that people want characters like Young Link or Pichu, who vary greatly in abilities and proportions to the original, to be labeled echos.
As for my choices, I think Shadow is most likely confirmed, and I think they will make him stronger but slightly slower than Sonic, maybe adding some fire attribute attacks to his kicks due to his rocket skates. I don't think he will get any chaos powers for his special attacs since no echo thus far has had different specials from their original.
Alph could work by giving him rock pikmin instead of purple pikmin. At the same time, they could make Alph pluck pikmin randomly like Olimar did in brawl VS olimar who plucks them in a specific order, making Alph's playstyle more random than Olimar's. He'd also be lighter.
A want Impa as an echo of Sheik, for no other reason bar the fact that the LOZ series needs a new rep that isn't a link clone, which we haven't gotten since melee. Impa is also the most recurring LOZ character bar the triforce trio. As for how she'd be different than Sheik, I don't think their would be much differences aside from super-minor changes, like how much knockback or damage an attack does or what direction they are launched.
True Midna could work as an echo of Zelda as well, but I think if anyone is being an echo of Zelda, it will be Hilda. Though I personally want Midna due to her being an original character rather than Hilda who is another Zelda like Toon Link is another Link.
As for other potential characters, that I don't care to be in or not:
Funky for DK
Demise for Ganon
Dark Samus for Samus
Chrom for Ike
Shadow Mewtwo for Mewtwo
Ninten for Ness
Overall, I think a total of 6-7 Echos are acceptable, maybe more considering Sakurai said we won't be getting too many newcomers, so Echos may be the best way to include more characters.