I think adding some extra complexity to night trolls would be the best way of doing this. Give them personalities and motives that would allow you to befriend one, and maybe some of them could wind up being fairly decent folk who were ostracized from society because they were strange looking, etc.
And I do like the idea of randomly generated potions and poisons, maybe even naturally growing magical herbs and fruits, they are a standard fantasy world element after all, and would be quite easy to implement under the current syndrome system.
Well, one mandatory tweak would be making them (at least potentially) non-hostile.
As for the brews themselves, they can be made in a process vaguely like cooking where two, three or four ingredients are combined. The procedurally generated part would be the mystical use (creature effect) for each ingredient, probably a decent positive and a mild negative for each. Similar effects in the same brew would stack, and Herbalist skill would give knowledge of the mystical uses.
Not sure exactly how to parcel out that knowledge, but it can be assumed that Witches (or night trolls) would have very high skill in this area since that is their ecological niche in the game. Something like:
- Famous ingredients from legend
- Common plants in homeland
- Common animal extracts in homeland
- Processed products (alcohol, golden salve, etc.) from homeland ingredients
- Uncommon plants in homeland
- Uncommon animal extracts in homeland
- Common ingredients in aligned-sphere biomes (for now, call that Good regions)
- Common ingredients in opposed-sphere biomes (for now, call that Evil regions)
- Common ingredients in unrelated-sphere biomes (for now, all neutral regions not in homeland)
- Uncommon ingredients in aligned-sphere biomes
- Uncommon ingredients in opposed-sphere biomes
- Uncommon ingredients in unrelated-sphere biomes
So for example, suppose strawberry seeds get the mystical use of speeding healing by 25% with a side-effect of dizziness for six hours, honeybee ichor gets speed healing by 25% with a side-effect of coughing for a day, and elk bird heart counteracts coughing with a side-effect of dizziness for an hour. A human herbalist could whip up some strawberry-honeybee potion for 50% faster healing but with a couple unpleasant side-effects. A dwarven herbalist whose homeland includes the caverns could throw in some elk bird heart to get rid of the coughing.
A simpler system would be to just generate a crap-ton of syndromes with 2 or 3 random ingredients.
In either case, the witch might send the adventurer on a mini-quest to get an ingredient, or one visiting a fortress could offer to make additional brews if provided certain ingredients. The demanded ingredients need not be related to the brew that the player is trying to acquire; it might simply be valuable to the witch and hard to acquire otherwise.