Perhaps give Barkskin a certain amount of Fire vulnerability?
And on that note give grease a colorbomb like effect that when casted on people they gain the shadowshield VFX along with a fire vulnerability.
Just some thoughts to change up the way risk/reward and defenses are handled.
I would pose that barkskin provide an equal amount of electrical resistance, if this were put into effect.
I don't think giving all grease spells fire vulnerability fire vulnerability is a good idea.
Perhaps adding it to the oil bottle throwables might be more appropriate however.
As for Barkskin....I don't think adding vulnerabilities to spells intended to buff you is a good idea as a general development strategy unless you start going through all spells and give them downsides where appropriate. Especially fire vulnerability is pretty rough when it is likely the most common element of all for elemental damage attacks (fireballs, combust, default of a fairly common level 2 buff 'flame weapon', animations, etc).
Fresh bark retains moisture so its not that flammable, its the first protection for trees against extreme temperature changes. Its dry bark thats flammable, and even then its not more flammable than the clothing may be wearing already
(seems i cant simply edit the previous post by phone easily)
What I saw on another server that I really liked a way back is theyd made the grease spell generate a damageable small placeable where if it got hit by fire the grease changed into a Fire Bomb throwable effect, swamping it all with fire for a (very) short duration.