Oh, I figured when you were talking about more interesting FE echoes that you were thinking of Celica. I saw her name thrown around a fair amount as an Echo of Robin. I figured that since her game came out in mid-2017 that she was too late for the Smash project plan in addition to not being as popular an option as Chrom (for the obvious reasons of Gaiden never releasing in the West haha).
Also, Three Houses?
Perhaps, but it was in development around the time. As someone noted, FE characters are generally promotional and the value of them is to create more fanbases, so giving SoV a wider audience could have been really good.
So far 1 of the 4 DLC fighters had a recent or upcoming Switch game, so it would be pretty impressive for it to get even less marketing-focused than that.
All retro pack maybe?
Hm? There's a King of Fighters game coming out, two of the four Heroes have games on the Switch, and Joker appears in Persona Q2 and Persona 5 Scramble. Banjo and Kazooie is the only character that doesn't have a game coming to the Switch.