Why there's only leeches and gauze that you can himself with? I suggest to make skill that will allow us to make mixtures (not only rustroot one) and other usable stuff from various ingridients. I see 3 categories of products of apothecary:
1. Potions and mixtures, not magical ones like in Diablo but realistic ones. Like mixture that will give you a buff with which you will regenerate hhp while sleeping or salve that will restore some of your shp instantly but will backfire after.
Or some powders that will increase quality of food or give to it new FEP (spices actually).
2. Poisons which will decrease quality of food or water, or/and after eating poisonous food heartling will be KO'd for some time. And poisons that can be applied to weapons for damage boorst.
3. Perfumes (its not actually product of apothecary but still ok i think). They will allow you to attract animals or scare them otherwise, or give some charisma increase. And while using perfume heartling will leave scent of perfume instead scent of crime.
As you may see apothcary can be very useful, and because its must expensive or time consuming. There must be many process involved like destillation, dehumidification, boiling e.c.