I think the shared animations also somewhat messes with the uniqueness of the zombie, since I (believe) the demolisher, the mutated zombie, and another normal zombie share the exact same animations. It sort of feels like all of the special zombies are either goo spitters, or exploders. Unfortunately, I'm pretty(?) sure spider zombies can't even climb up walls anymore, which sort of amplifies the problem.
Generally I don't come up with good solutions, so take a massive grain of salt, But I'd enjoy something that can be responded to by players. As it stands, I find the current acid pools frustrating, and the animation is no different from a zombie stun animation, and the cop acid pools do no damage, but these do, so I'd personally do away with acid pools altogether. My idea involves them charging/telegraphing for an attack which releases a toxic mist when complete. This would buff nearby zombies with increased speed and block damage. If they're hit during this chargeup, the attack would be interrupted, and put on a cooldown. When used, this attack reduces the zombie's HP by 50%, so they can't spam it too consistently, or stack too heavily; potentially killing themselves in order to bolster a horde. Maybe fits the idea of a mutated abomination that does something unexpected. A slight green haze/trail/particle effect around the zombies' heads could denote the buff, making it more clear to the player which zombies are buffed and which are not.