TLDR: if it seems like it's x, make it weak to x-bane gear, but implement specific enemy susceptibility to balance and preserve metas.
We've all been there at one point: "What do you mean salve amulet doesn't work at barrows??", or something equivalent. There's always some poor rational why it doesn't work ("They're wrights, clearly not undead!"), but we all know it's just about game balance. With recent updates to the "Demon" status giving it different effectiveness on different enemies (Duke having only 70% bonus damage, while Yama has 120%), why don't we apply the same levers to other attributes to allow thematic weaponry to be viable without being game breaking? Here are some examples:
-Skeletal Wyverns (undead)
-Barrows Brothers (undead)
-Ba-ba (undead)
-Vardorvis (undead / vampyric)
-Zulrah (draconic)
I believe this could be a good project for the summer sweep-up. Having intuitive enemy weaknesses is an important part of any game.
Edit: Had an additional section on slayer task thematics, but decided it's best to limit the scope of this post to the less controversial stuff.
Additionally, there are many monsters which thematically could fall under certain slayer tasks. These would be more controversial to balance as there is no precedent set for varying effectiveness from slayer helm boost; if you see an xp drop after a kill you immediately know the 15% damage buff is in effect. Perhaps a blanket -x% effectiveness could be applied to monsters within raids if meta shifts are a concern.
-Zebak + minions (Crocodiles)
-Ba-Ba + minions (Monkeys)
-Soteseg (Dark Beast)
-Pestilent Bloat (Zombie)
-Nylocas (Spiders)
-Vardorvis (zombie / vampyre)
-Revenants (currently all count as "ghosts", but could fall under their respective tasks too)
Additionally, I think wolves and dog tasks should be merged into one "hounds" task and include terror dogs and hellhounds.