As far as what the game already has for poison effects I would keep them, though they probably need reworking done to have greater numbers. I think it would be better to introduce entirely different and out of the box concepts for poisons. For instance one that blurs the players vision (and on npcs would reduce their chance to hit for similar results). Poisons that slow movement/attack speed, or increase them. Maybe even greek fire (might be out of your scope to produce but it would be cool) There are plenty of things poisons and potions could conceivably do that are not in the game but the engine is capable of producing, and they dont all have to merely be tied to health, mana, speechcraft, ect.