To be honest, I have trouble coming up with some Likely Echoes
Most of the Potential echoes I see people throwing out, Likely wouldn't get in anyway as echoes since either
-They would probably get in as their own characters, since they have the merits to (Shadow, Dixie, Maybe Tom Nook and Octoling)
-Their Base character aren't in yet, so it's hard to judge (Coco)
-They Could just be alts (Dry Bowser, Gooigi)
-They might not have the merits to be in as an Echo
the best 4 I came up with were
-Mrs. Pac-Man
-Jeanne
-Ninten
-Black Shadow
Mrs. Pac-Man has rights issues, but they might have also been sorted out since then. Also to be honest she also could just be an alt as well, though if you want to have the experience of Mrs. Pac-Man, you could make her Faster, but perhaps more vulnerable, to match how her game was Faster, but harder as well
Jeanne has been passed up in Ultimate as well. Maybe if Bayonetta 3 actually starts existing and she has a major role in it, then perhaps she could be Echo instead of just a Recolor this time
Ninten was the OG Protagonist, so that's something to consider. Nintendo has also been showing love to Earthbound it seems in some aspects, but unsure on this one
Black Shadow is a weird case, since I know alot of the requests for him come down to Having Ganondorf's old moveset when Ganondorf gets that new moveset, if that day every comes. I'm unsure if that would count as an echo or what though
But I honestly have trouble thinking about who would be a Realistic Echo. I think there is a good reason why there were only 5 New Echoes in Ultimate, and why there were none in DLC