Herbalism as I understand it is related to mixing things.
One suggestion is to implement mortar and pestle and to allow us to mix one ingredient with another to produce an edible/drinkable item, for example, mixing birdcherries with juniper berries and willow catkins.
If the recipe is demanding enough (e.g. 20 birdcherries, 30 juniper berries, water), this would give the players reason to value currently useless trees. Right now people only plant those for their wood color. The crafting formula could also give much more weigth to water quality.
The resulting items could give temporary and balanced stat changes, for example, -6 agility/+3 strength, or +8 perception/-5 constitution. Particularly exploitable recipes would give temporary HHP penalties as well, suggesting that the product is too strong- poisonous even.
In fact, it would also be interesting if all of the recipes gave stat bonuses in exchange for HHP, which would not only make constitution an useful stat again, but also show that in order to meddle with experimental stuff like this you must have a strong body. There would also be no categorical difference between "medicine" and "poison" and I like that concept.
This feature would be very welcome by hermits, who have only one character and must to struggle between several tasks with it. I'm sure it would be fun for combat too. There's a precedent for this too, as in w7 alcohol would temporarily increase and decrease some of your stats.