Personally, My thoughts on some of the Popular Echo Suggestions
-Dry Bowser and Mrs.PAC-MAN honestly work better as alts. Especially for Dry Bowser, as there's no Different Characteristics that separates Dry Bowser from Regular Bowser
-Octoling could work. I Imagine that with how Big Splatoon is, Octoling could be a Semi-Clone in that they specialize with Weapons from 2 and/or 3 (as Inkling is focused on Weapons from Splatoon 1)
-Ninten does have his unique abilities, but first he needs a redesign, but also Mother I would say might not have enough priorty for Ninten to get his Skills. I'm not sure
-Jeanne could Probably work, though to be honest with how Volatile Bayonetta's Moveset design is, I could see why they haven't gone for it
-Dixie Kong, Impa, and Shadow could theoretically work, but I actually imagine that they'd likely get in as their own characters over being echoes
I think when people consider Echo fighters, they only consider if the character has a similar body type and perhaps the ability to do the moves another character to do, and just assume they are Destined echo material, when there's more that goes on
Echo Fighters still have to go through the Merit checklist, and typically Most of the echo Fighters might lack in some area of the merit checklist that being an Echo Fighter is their only shot
Daisy, Lucina, Dark Pit, and Chrom I imagine didn't have enough distinguishing moveset ideas to be implemented as Full Fighters. In Chrom's case, this was specifically expressed by Sakurai officially, as he couldn't come up with a Concept that wasn't just mix of Marth and Ike.
Dark Samus is a very irrelevant character, and While Sakurai did say she was requested overseas, I can't imagine that it was big enough to reconsider, especially since Ridley heavily overshadowed her anyway.
Richter and Ken are special cases, as Richter and Simon share a Moveset, and Ken pretty much has his unique things in Ultimate anyways. coincidently, they happen to be 3rd Party
Dixie, Impa, and Shadow don't really have any Drawbacks in the merit department. These characters have been requested, are Relevant, and have More concrete moveset Ideas that can be implemented when you compare them to the Echo fighters. Dixie's Spirit as an example suggests that Sakurai still wants to Implement that Duo Mechanic from the lost archives of Brawl, as Dixie's Upgraded (4 Star) Spirit is her with Kiddy Kong