Alright, so I've been wondering if this is possible, and I figured if anyone can give some valuable input it's the people in this thread.
I know many are saying that the ARMS character could be several of them as alts, and there's been some pushback because many don't feel that the characters could be done justice, personality and gameplay-wise, with just alts. So that got me thinking... Could we get them not as alts, but as Echo Fighters?
I know many will dismiss the idea immediately, I get it, people aren't expecting Echo Fighters period, nevermind 3 for a single character, but try to get past that and see if the idea works. I believe that Sakurai always tries to go above and beyond, and I think what I came up with is a good way to represent the characters while still being very much doable.
So, the four characters are the ones you'd expect: Spring Man, Ribbon Girl, Min Min, and Ninjara. I agree that they're the ones that make most sense, they're all the closest to each other proportions-wise, and while others like Kid Cobra and Lola Pop could also be done with the Echo format, I think more than 4 is too much, and Ninjara and Min Min are the popular ones. They have different voices and animations (which isn't new, Inkling and Byleth do it), while sharing attacks.
Spring Man is the "default", but it's kind of like the Belmonts where they're all Echoes of each other. For example, everyone has kicks. The differences, besides in the animations, comes in the fact that everyone keeps their unique mechanics. Spring Man has his arms autocharge at high percentages (say, over 100%), Ribbon Girl has 4 jumps (and maybe double airdash, but I think that's quite an advanced mechanic to introduce with an Echo), Min Min has her dragon arm mechanic (by charging or by grabs, and it goes away when hit with enough knockback or another status effect like stunning), Ninjara can teleport while dashing (giving him more movement, especially while airborne, and more invincibility frames).
If that seems like too much, I could see Spring Man going without his mechanic, as it steps a little bit on Terry's toes and keeps him from being the simple, easiest-to-get-into of the bunch. I could also see Min Min going without her mechanic because her kick is already present in all the characters - and it's fitting that the two "default" characters be the main character and the most popular.
If arm switching is a mechanic, and I could see it being one, then they could all have their three default types. The only one I can think of that would pose an issue is the Dragon, because it's pretty complex - maybe that could be replaced with a multi-shot for balancing reasons anyway, as the other 3 have one as well.
So, one character has Terry's mechanic, one has extra jumps, one has better dashes, and one can store charge for attacks. That, plus Chrom-level animation changes at most. Suddenly doesn't sound like quite that much, does it?
Well, I dunno. Let me know what you think, if it's plausible for us to get something like this come June, if it would explain all the mystery around the character, and if this level of representation for each character would make you happy.