I think the definition of echo is pretty loose. Some are exactly the same but with different animations, Chrom has an entire different move, and Ken has multiple different moves and is faster and weaker iirc. There's no reason Metal Mario wouldn't count if Ken does. The metal box I didn't think of though. I don't play with items so I forgot that was even there. I guess it would just make Metal Mario even heavier. The character wouldn't be Mario with the metal box, it would be a character with it's own base stats. It should work the same way it does with any character.
Stats aren't actually loose. With the exception of Ken having a different speed a bit, everybody has the exact same stats. Ken is treated as "a fairly big exception" and even compared to Dr. Mario as he borders being a regular clone over an Echo. Dr. Mario is also noted as a clone who borders on being close to an Echo. But still holds the same weight value and most of the same stats. It's not a good example to use stats as loose when they blatantly aren't for Echoes.
Moveset is the only thing that's loose, since it's more about "how easy is it is to make the moveset work from the base or borrow from other characters". Dark Pit copied Light Arrow, for instance, so easy to program. Chrom's main unique move is borrowed from Ike, etc. These aren't what I'd call "major" differences in terms of development requirements. Animations still require obviously more than an alt costume, of course, but they aren't nearly that loose. The big thing Chrom and Lucina even have is removing the sweetspot mechanic. So basically Giga Bowser is actually extremely close to what an Echo is other than stats, as every move is basically a revision of Bowser's. Metal Mario obviously is bar the stats, being an exact clone of Mario(albeit, he's only an actual character in the N64 game, but he'd probably just be Melee Mario at best? Or at least focused to be akin to that. Dr. Mario in a sense has the whole point of heavier/stronger than Metal Mario has, while still having his own clear attributes too).
Even then, movesets in most cases aren't actually loose outside of Ken. There's usually one minor or big changed move, sometimes a mechanical change(Lucina) that overall affects the way they fight, but still shares the same overall moveset as their base. I don't even need to go into bad ones like Daisy who doesn't have anything different besides a hurtbox on a very select few moves/animations/taunts. -_- Dark Samus has electricity only but the moveset is pretty much the same otherwise. Richter has one slightly different move(which rarely comes up), and I'm not sure if his hurtbox changes for some animations among taunts/idles/etc.
In the end, Ken is the only one who plays loose with how Echoes generally are. He's not the normal one. Lucina and Chrom are more or less kind of the most neutral in terms of changes. Same stats, slight change to how they play, sometimes a new move. That's the usual Echo thing is they aren't almost identical(see Daisy for a case of "99.999999999999999% the same. Richter is more like 95% due to one move being relevant rarely).
Not sure why I didn't specify this before, but I thought it would be best to make his Metal Box form the N64 model. I think it would play out like this:
- Mario and Metal Mario would be different because one is metal and the other isn't.
- metal Mario and Metal Mario would be different because the former is much less shiny than the latter. (Metal Mario isn't very shiny in his renders, but his models in Mario Kart are super shiny.)
- metal Mario and metal Metal Mario would be different because entirely different models.
- The same would apply to Metal Mario and metal Metal Mario.
Dunno if that's exactly how it would work in practice.
But then how do you tell Metal Mario and regular Metal Box Mario apart if they're almost identical otherwise? Cause I thought the point you were making is Metal Mario wouldn't look different enough from the Metal Box form. I know there's the iris stuff, but these are very difficult to tell apart. Slightly different hues are bad as costumes too, so...
That said, it's a cool idea if you can already tell Metal Mario and Metal Box Mario apart without the N64 model first. I'm fine with whatever works~
Or did you mean Mario turns into his N64 Metal Mario model instead? If so, what does Metal Mario gain from the Metal Box? Cause that's where I think a problem arises.