To fill the thriving role, I propose two enemies.
The path of metal is dangerous, and runneth over with Posers, who would steal the tools and look of Metal for themselves for only fleeting use. This cannot be allowed, so it is the duty of every headbanger to crush them into paste. Act as skulkers, so you may or may not have run ins with them.
But posers are naught compared to the likes of the Hairbangers, practitioners of the oft ridiculed ways of Glam Rock and Hair Metal. While well armed and true in devotion, they often steal from their surrounding neighbors, and their ways are gaudy and atrocious, and must be corrected by those who wield the power of true metal. Nonskulkers, they will come to fight you eventually.
Additionally: Magma workshop called of course, The Mouth of Metal. With the power of the mouth, the most terrifying growls and ear splitting screams may be learned by those who truly follow the Voices from Beyond. Could potentially result in minor combat log spam. Can provide bonuses to allies or penalties to enemies. Got some ideas below.
Growl of Death (Offensive): Unleashed with the greatest ferocity, a warrior unleashes the power of a Death Growl, often stunning enemies with the intimidating display of voice-work, and potentially paralyzing others with fear (stuns targeted enemy for a week, and may paralyze them completely. ideally, total paralysis would be resistible based on disease resistance or willpower.)
Voice of Metal (Offensive): Letting loose a loud and proud Metal Scream, enemies move with less overall vigour in battle, feeling depressed and inadequate as vocalists and warriors compared to the user of the Voice (Interaction halves enemy unit's agility and willpower.)
Call to Arms (Defensive): The vocalist's comrades are inspired to fight with greater haste and vigor by the power of his or her call to arms. Buffs ally agility, endurance, and willpower for a week.
Scream of Darkness (Offensive): A harshly delivered and evil metal scream or string of lyrics deafens enemies, makes them bleed from the ears, and suffer debilitating pain. Stuns the enemy for three days, and causes bleeding from the head and pain.
OTHER STUFF - If we could properly implement instrument weapons, specifically guitars, such as the guitars being made of custom materials or something granting interactions via itemsyndrome.
Facemelter: A guitar solo of epic proportions, that destroys the faces (and minds) of the guitarist's enemies. Causes skin and brain necrosis in one day (practically instantly in fort mode.) Ideally just head skin rots.
Wrath of Metal: Riffs played with such power and speed that the notes have come to life, and strike the enemy hard enough to pulverize anything they hit. Material emissiona ttack that hits with so much force it pulps the target, armor be damned.