Kinda off topic but I'm wondering what the differences between Toon and Young Link will be to justify not having one Echo the other, but that might also be more of a semantics things since Toon Link never directly clones Young Link.
Toon Link doesn't have the legs to do the usual Link kicks in his moveset. (Otherwise, Young Link appears to be more of an evolution of the original Link moveset while Link and Toon Link have themselves changed by the time Young Link could come back. Also: different proportions so simply pasting the Y!Link model over the Toon Link skeleton is a no-no.)
Like, here's some notable differences:
Dr. Mario
- Mario's Mario Tornado became his dair when his Down Special became the FLUDD. Dr. Mario kept the og dair and his Tornado is still his special. That his moveset is different from Mario's because Mario got changed is more than enough to differentiate him and let him keep his separate roster spot.
Young Link
- BOTW Link has remote-controlled bombs now. Toon Link can't do anything using his legs because they're stubby. So similiar to Dr. Mario, Young Link is different by essentially staying the same as he was. Also, different proportions.
Pichu
- Probably has to do with his proportions as to why he is not an Echo Fighter. Pichu's proportions are different enough that simply pasting it onto a Pikachu skeleton is... probably not gonna work. Besides that, we do not know his whole moveset, so there's possibly other changes lurking behind his inclusion as his own fighter.
Compare that to the Echo Fighters who use what's basically the same moveset with only statistical changes, and have similiar proportions to their parent characters, and it's pretty clear why the above mentioned three are treated as actual separate fighters while the Echo Fighters are treated as... well... echoes of existing ones.