I wasn't even really specifying coverage wrt visual progression, heck I don't even see anything from steel to rune covering each other any more or less than the other, that problem's restricted to bronze and iron. I just think that different metals would need different processes and techniques to properly shape them, alongside them needing different shapes overall in order to meet a desired outcome, e.g. a bronze sword would need to be differently shaped from an iron sword on account of different strength, malleability, etc. Heck, runite has lore that specifies that its toughness is in part due to it generating a magical membrane on its outside, which should feed into how a smith shapes the metal and what they shape it into. I think metal armors in Daemonheim were originally designed with that in mind, but they also suffer from some level of sameyness between certain tiers (though that can be explained away with all the metals having the same taint of Daemonheim that leads to less needed differentiation in their shaping).
But with game identity, I can't fully agree with "face cover and plume makes it visibly identifiable as RS." That's oldschool. Same origin, different game. Sure it's recognizable, especially being the same style of RuneScape that everyone who's played RS grew up with, but even if oldschool is more popular than RS3, I don't think we should seek to implement certain stylizations on the basis of recognizability of something else. I can agree with ensuring some form of consistency, but the popularized retro plate reversion would lose a lot of visual identity of each tier of equipment. Same would have to retroactively apply to orikalkum and above.
Plus, it'd make their cosmetic override releases in Marketplace pointless. Not that that specifically would be something to avoid, I mean it wouldn't be too bad to me if they were suddenly made free or ingame items separate from their originals, but I just think it'd be weirdly specifically cluttery and a little unnecessarily confusing if we have the ingame item of a Saradomin platebody not only be identical to the Marketplace Saradomin platebody override, but also the ingame version still be able to be keepsaked. As one example.
however, I will agree to giving ALL full helmets plumage, as well as making the current bronze and iron full helms into their medium helmets and visually make the full helms face guarded (of course, not with the SAME face guards). Less of a recognizability thing and more of a rule of cool imho. If that does make it more recognizable as RS somehow, then sure, let that be a bonus.