Alchemy is already in to some extent, there's the profession and phials and a potions category...
I imagine potions like Speed, Berserk, See Hidden Creature, Invisibility, Invulnerability maybe, possibly Gain Experience (in a random weapon skill) ... each with a (huge) chance of failure according to its quality. (90% failure for a normal potion, 40% for masterwork, 0% for artifact?)
Potions would, in order to be rare and difficult to get, need different herbal ingredients: Specific alcohols (drinks) for the distillation, rope reed or GCS silk 'filters' made by craftsdwarves.
The ingredient quality would affect the potion quality, so only well-run fortresses would eventually get powerful potions.
Some of the more potent potions might require specific gems; and ingredients from different biomes. Rare potions might require plants from opposite biomes, e.g. from both good and evil biomes.
(e.g. Cheap potion: any filter, strawberries, dimple cup.
Difficult potion: Hematite, Obsidian, filter from GCS silk dyed with sliver barb, plus Dwarven Rum, Sun Barries and Whip Vines.)
... however that could a) be frustrating ("Nooo! Don't brew those 5 Whip Vines! Stupid dwarves ...") and b) result in players spending even more time on looking for the one site that has it all (sand, water, magma, hfs ... it's a daunting enough task finding a new site without referring to a list of plants per biome, and worrying about which potions the fortress is going to have.
One kind of potion I did not include in the list ... How about healing potions? Finally get those starving, dehydrated, yellow-limbed champions out of bed and back on patrol?
Hm. Sounds too good, doesn't it?
How about making Potions of Healing / Limb Restoration available only from trading with Elves, but not from killing the elven traders? *grin* *duck* *ouch*
[ May 19, 2008: Message edited by: Samyotix ]