Welcome to the Duke of Death Hot Take Power Hour, Please Apply Tin Foil Cranial Protection and Don't Forget To Stretch For Reaching Purposes:
A lot of people are convinced Dixie is gonna happen, and I see her highly likely at the very least. IF Dixie happens, that would have been the third series so far that we've gotten a Newcomer and a Echo for (Castlevania and Metroid). That would establish a pattern. Full on newcomers coming packaged with echos (but not so much the other way around, lookin at you Chrom).
People also think Skull Kid is very likely, as do I. IF Skull Kid happens, its not entirely impossible that LOZ would get an echo accompanying him. And since Shiek got her final smash changed, something less Zelda specific, I wouldn't be surprised if that echo would be Impa.
Now, we do have another new echo being Daisy. IF Dixie happens and IF Skull Kid with Impa happens and IF that establishes a echo/newcomer pattern then I think it'd be rather likely that a new Mario rep, Geno perhaps, could be the corresponding Newcomer to Daisy's Echo.
IF