I was wondering if any of these things could be possible right now:

Damage-altering states
Throwing something onto an enemy would make them more susceptible to damage (a la Jarate from TF2).

Oil could be made more interesting by making the enemy more susceptible to fire damage. Could lead to a need for more useful weapons... maybe Fire Arrows? Because honestly the only ranged weapons I personally believe are useful are the Ballista Xbow and the Catasling.

Slows
Following the idea of coating an enemy in something, coating an enemy in some sort of goo (honey?, lol) could slow them.

Or, attacking the legs of an enemy could slow them.