I'm not talking about multiples of the same character. I'm talking about Metal Mario and Metal Mario - you know, one Mario who is metal, and the other who is a Metal Mario.
Those arrows disappear when the camera is at a certain distance - what happens when Metal Mario the Mario and Metal Mario the Metal Mario cross over at close range?
This is why the rumour of a couple of years ago was silly, and it's been demonstrated beautifully in text form here.

There isn't really a simple way to have a character of a series' regular power-up be quickly and clearly differentiated from the power-up being used by the character that's been cloned in a non-confusing fashion.
Pretending that Metal Mario is a character is something that works in the Super Mario series and its spin-offs because, as
SuperMii3D
noted, the Metal Box power-up has been gone from there for over a decade. This breaks in Smash.
At a glance, that won't be good enough - you're designing for a "ten-foot user interface", in this instance, and a small animation change is user-unfriendly in a case like this.

And as long as the Gold Plains stage remains, gold is a no-go, because that would become very confusing very quickly.