Dark Samus switches other elements to electricity. That's an actual gameplay change that's noticeable.
Eh. It's not
that noticeable. It really just means she can't outright destroy bombs and can damage Red Pikmin, which are both still pretty niche interactions. Also extremely niche, but her Charge Shot hits lower, allowing the uncharged version to hit Pichu sized characters more easily.
That said, Sakurai didn't really define them beyond "they must be the same bodyshape" as is. Which despite being one small rule, is clearly ignored a lot. No, they aren't "a typical clone", they're actually quite different from how they often were(which were major differences and bodyshapes). This is why suggestions like Black Knight doesn't really work since he's got a different overall size. Likewise, this is why Black Shadow can't work for Captain Falcon. They are not the same body structure(they have quite a noticeable difference). Even when Lucina had a tiny height difference, Ultimate changed it to her being the exact same bodyshape to keep this rule intact.
Similar =/= same. The only thing similar even allows for is finding moves among other characters that can easily be retooled. Like Ike is close enough to Chrom's bodyshape that you can retool the Aether into a new move, but Chrom's bodyshape specifically matches Roy.
The only other thing we've seen is that they always have the same weight, and usually their attributes are about the same or not that different(Ken is an outlier). Regardless of how they do said attributes. Though on an interesting note, Sakurai put together a team to balance all the characters. This could explain why some have barely noticeable differences(I.E. Daisy), because as long as the actual character functioned without issues, it's not a problem in itself. Thus, there's no need for Sakurai to ask to change it up, because it works. When Daisy had a sudden difference, this was reverted due to it being an oversight(as well as some bugs). As much as it sucks, it goes against the original design. She was meant to be 99.999999% the same as Peach, essentially.
I think the main thing is that people just don't like standard Echo Fighters, and want to know how far they can push the concept because what they seemingly
could be is something that they'd want.
It's established that they need to have the same proportions, except there is some leeway because this isn't true of Marth and Lucina. So then, Black Shadow
might work should giving him Captain Falcon's proportions not cause things to look weird. Oh, and the height difference thing with Marth and Lucina is very strange, as since Lucina was originally a costume, she had to have been shrunken down after they gave her her own slot. It's not that impactful a change, and everything scales based on characters' models so did they change it tom be consistant with the others like you said or were they just correcting an accident they didn't notice in SSB4's development?
There's also weird hypotheticals with Villager and Isabelle. Isabelle can't be an Echo Fighter because her perportions are very different compared to Villager, but that's only because Villager uses his design from Animal Crossing: Wild World, while Isabelle uses her design from Animal Crossing: New Leaf. If Villager was also from New Leaf would Isabelle be an Echo Fighter? Their proportions aren't exactly the same in that game, but as we've established, they just need to be similar enough to be made the same without issue. If they did become Echo Fighters, would Isabelle be less different from Villager than she is now?
How many changes can you make to a character's moveset before it becomes too much work for an Echo Fighter? Most of them say basically none at all, having at best some different, mostly aesthetic properties on some of their moves. Chrom and Lucina have sweeping numbers changes to their attacks, and Chrom even has a new up special. This is probably not applicable to all Echo Fighters though, so what else is there? Well Ken has several new moves, and a few movement property changes that allow him a more aggressive variation on Ryu's play style, but it's so out of the norm that people often dismiss it. Then there's Dark Pit who has two variations on Pit's Special Moves that are impactful, but it doesn't change his overall playstyle much, and people aren't super happy with it.
The result is people suggesting Echo Fighters to add characters that woudln't get in otherwise, but wanting them to have actually impactful gameplay changes, and whether or not those changes can be made is pretty hotly debated because half of the people see the opportunity to add more Kens and the other half only see more Daisys.
