It's obvious that black metal has problems:
(1) It's too easy to fake.
(2) Its aesthetics and ideological surface are too easy to emulate
(3) Its scope is somewhat limited, emotionally, aesthetically and compositionally
The first two tell us: it's not enough differentiated from rock music
The third tells us: it needs to expand topically, thus driving an expansion of moods and thus techniques
In particular, how about some long melodies? Best examples are Summoning, later Graveland and Ancient (Det Glemte Riket
How about some through-composed, evolving, narrative structures like Burzum "My Journey to the Stars"?
There's more than can be done in the genre, but not if it stays three-riff verse-chorus even in the slightest.
Classic Hollywood music and 1930s pop had melodies that evolved, complexish songs, etc.
Progressive rock is very similar.
If metal stopped being all in the same mode (and mood), it could be more narrative.
Of course, it would lose that 100% negative intensity
But how long can you listen to that?