Ideas for mundane special attacks/abilities:
Ability to consume a living target whole
More venoms (We have numbness which reduces speed. we already have systems for pain, fatigue, nausea and bleeding, so why not have extremely painful venom, venom that increases fatigue generation, venom that nauseates, and venom that makes bleeding last longer? Also, a venom that causes extra wounds to other body parts while it's in your system)
Separate optional knockback value for attacks that overrides the damage-based value?
The ability to disgorge acid is quite possible.
Ranged natural weapons
Bristles that hurt grappling opponents
Ideas for not so mundane abilities
Ability to pass through walls
Other breath weapon damage types as mentioned before
Aura effects (raises the dead, randomly sends toggle signals to mechanism-linked things, fear, attraction)
Ability to alter object composition. Changing all weapons that hit the creature into ice, for example, which then melt away in the summer heat.
Blood types that create tiles of various liquids at 1/7 depth, or at more upon death (is this already possible using the corpse flag?), such as water, lava, acid, etc.
Ability to simply destroy objects made of some materials (wheee, fire-immune rust monster versus your magma system)
Oh, also something that JUST makes whatever it targets more and more unhappy.