Ah so it's a one-off ability or something he can just do all the time. Likely the former since why would he not solve the plot in 10 seconds otherwise.
To renote my edit, it also leaves him unconscious, so he never releases his full power outside of "a time of need".
The first time we aren't sure if he is left unconscious is during 06, but he's with his friends, and considering he's trapped in a capsule in the future, well... it's implied at least.
Yeah I think he’d be cool, and that sounds great. I thought Metal Mario would look different, too, even if he may have some shine.
About the moveset, tbh I only came up with some small differences, as I wasn’t really sure what unique moves to give him aside from the 3D Mario ground pound and I guess the metal sheet could work a bit different from the cape and doctor’s sheet. I like this idea of him being a sorta mysterious “reflection” clone of Mario, too, or whatever Nintendo thinks he’s supposed to be. I’d love his inclusion as a fighter/more-unique echo fighter, too, which is why I thought I’d share the idea, though I know he’s very unlikely even outside of DLC...
Well, he first got separated in Smash Bros. 64 and Mario Kart. Those are areas to look at. You could also use some of his other Cap abilities too. Change the Fireballs to a metal aesthetic as is. The spin makes sense somewhat, but his Forward B even could be a remake of Luigi's, with him jetting forward while wearing the Wing Cap(think the Superman Push that Raiden has in Mortal Kombat) for a more unique take on it. Could also become a glide state while using it in the air.
Being it's still a form of Mario, you have unique taunts. You could return the size growth one, despite it being... admittedly a bad taunt. You could throw in the Invisible Cap for his Air Dodge and Side Step as is. I'm not sure what you could easily replace the Fire Punch(Mario's Forward Smash) with element-wise. Other than a bigger Metal punch to get the idea across I guess? As noted, weight is a big thing. He's a lot stronger, but has way less jumping power(no different from how the Cap is meant to work). He could barely swim, but wouldn't get a drowning animation and would sink very easily. There's a lot that separates him from regular Mario in that way. Besides being cockier.
(I won't deny Paper Mario has more potential for uniqueness, heh. But I love both versions as is. They're both neat in their own right~ Paper Mario is having an issue of too many abilities now, so toning it down to one set is not easy).