We dont have:

Slavonic metal
Nusantara metal
Oriental metal
Mongolian metal

Just a suggestion, since we have Viking metal and Celtic metal on a separate genre classification despite these 2 can also falls under folk / pagan metal category.

But if you'd ask me, those 4 genres which i mentioned above, also have their own unique identity as how viking and celtic metal does in order to be named under a new metal genre.

So why not?

There isn’t even a consensus that viking metal is a legitimate genre (I don’t think it is). Why would we want four other nebulous tags?

What would “Nusantara metal” sound like anyway? Brain Ass plays brutal death metal, not “Nusantara metal”.

Viking Metal for me is a more first-wave black metal and thrash influenced approach opposed to the second wave/modern black and death metal approach of pagan metal. And Celtic metal is a combination of Celtic Rock (which is a genre) and (Folk) Metal afaik hence a somewhat legitimate genre while the stuff you suggest is basically Folk Metal with differen folk instruments.

Well,

Windir is listed as Viking metal: https://www.metal-archives.com/bands/Windir/360

as Heidevolk: https://www.metal-archives.com/bands/Heidevolk/17850

as does Midgard: https://www.metal-archives.com/bands/Midgard/3540417584

and yes, ENSIFERUM: https://www.metal-archives.com/bands/Ensiferum/332

Despite the fact that these 4 are also basically folk metal with folk instrument. But they get the honor of being classified as their own genre in MA.

Brain Ass is not nusantara metal, folk metal hell no, not even pagan metal.

But this is the early proto-type of nusantara metal: https://www.youtube.com/watch?v=1C4pSnPZ5oY&t=1s

Basically Nusantara metal is a pagan / folk metal which uses Nusantara traditional instruments (various instruments, like the one on the link above uses Gamelan & Gong) also emphasize nusantara guitar scale like example Javanese scale, Malay scale into their riffs.

It's weird indeed. Even the term "unblack metal" was ditched because it had nothing to do with the sound, just the lyrics. As far as I know, there are no viking metal bands that don't sing about vikings or norse themes. (And no viking metal bands that couldn't be labeled as epic heavy metal, melodic death metal, folk metal or combinations thereof.) It's just as silly as "pirate metal" or "nusantara metal" mentioned above.

It would follow logically that all folk metal bands should have their own genre based on the branch of traditional music they incorporate into their music and/or the theme of their lyrics. "We are Siedi Satans from Brazil and we play Skolt Sami Reindeer Herding Metal!"

Viking metal is a distinct style developed by bands like Bathory and Windir. It is not just singing about Vikings. Fucking Amon Amarth.