A few:
(1) Ambient metal in the Darkthrone
Transilvanian Hunger/Immortal
Pure Holocaust style: break up the drum phrases, emphasize bass drum over snare, make it more like a pulse than a "beat" and thus de-emphasize the verse/chorus distinction.
(2) More like Summoning
Dol Guldur except more variety in melody and perhaps even less of the traditional black metal and more of a new genre that doesn't fall into pop/folk imitation like
Let Mortal Heroes...(3) Not metal, but why didn't Vaesen make more albums like their first? It's godly. The rest try too hard to be rock music, which they aren't.
(4) Deeds of Flesh
Path of the Weakening -- what made this great was its inclusion of melody and seemingly random song structures that still came together. They never quite hit that point again. Why no one else?
(5) Who inherits from Suffocation?
Pierced From Within was brilliant but reined it in from the second album, which really had a lot of undeveloped potential.
(6) Phrasal death metal. Early Massacra, early Incantation, and Morbid Angel
Covenant showed us the power of this kind of riffing. How did it just about entirely die out?
(7) Ancient
Trolltaar showed metal how to build longer pieces that were not entirely dark, but had varied moods like an opera or theatre production.
(

Why haven't metal bands gone ancient Greek (no, not
that way) by incorporating drama, poetry and music together into one artform?
I won't ask varieties of "the sell-out question," such as why Trey and Dave are dicking around with an artform that will never reveal their talent, instead of making complex death metal, or why Gorguts has a hardon for deathcore.