I think 1 thing we lack here is the use of the alchemy skill...
I dont say make Gheal pot heal for +200hp, str pot giving +12938021389 str bonus for 24h, but some stuff could be made whitout being too OP. I also see more works for player (quest, ect) to improve game play
Tolerance
First, for all good system, there need to be restriction. I see 2 ways I personaly like, first a kind of cap, that we could call tolerance to the intoxication (in real life, the effects of plants and drugs humans use are a reaction to the toxics to the plants). The human body create a tolerance to the toxins in the drugs. making less effective the more you use.
The way I see it, is add a kind of check. All player could start whit a max tolerance lvl of 50. let say you drink a Gheal pot, it heal you for XXhp, and add 20 tolerance. your tolerance is now at 20/50. every few second, you loose 1 tolerance. The more tolerance you have, the less the pot heal. Let say a Gheal heal for 100 hp. you drink one at
0/50 tolerance and it give 100hp (maximum effect)
10/50 tolerance and it give 80hp
25/50 tolerance and it give 50hp.
Here is the calculation I used : [size=78%](tolerancelvl x10 % by mtolerancelvl = % reduction in pot effect)[/size]
This would also allow you to add a way (rare drop on a boss/champ) to get more maxtolerancelvl, whit a cap (75?) so higher mtolerance = lower effect reduction on pots. This system is to lower the pot spamming, since the more you use, the less effect, and waiting more between pots increase the effects. This could only be applyd to heal pots, but even on +bonus pots (if scripter feel like doing it xD)
Also, adding rare reagan droped from some places (special mobs of a quest whit multiple stages?) so the special pots cant be crafted unfinitly. The more "usefull" ingreadiant being rarer.
Kind of pots could be +stats/effects (Hit effect?, Reflect, stats, mass paralyse (kinda like Volcano from Druid book but whit modified effects), HoT, Self res, etc. all whit a "good timer" (nobody like spamming pot every 30-60s to keep effects)
The reag could be added a little like how Zelda quest work; get me X item (reag) from X mob, and it give you location of next item to get, whit a special boss at end (champ/peerless kind?)
Anyway, just an idea, as usual 