Kibou: For example, there could be a potions that helps your body to deal with the poisonous effects of other potions, reducing the total toxicity you take. Or one that increases the positive effect of other potions at the expense of additional toxicity. I'd especially like one that makes other potions last longer.

Kibou: I've got an idea for future DLC, expansion, Enhanced Edition or whatever. It's intended to add more depth to alchemy/potions, and fill the gap that was left by the secondary substances that didn't get carried over from the first Witcher.

The interface that is used in The Witcher 2 for drinking potions is based on drinking multiple potions at the same time - you select three or four bottles, then gulp them down in one go. So wouldn't it be nice to have some "supplemental potions", potions that have no effect by themselves, but influence how other potions you drink at the same time affect you?

For example, there could be a potions that helps your body to deal with the poisonous effects of other potions, reducing the total toxicity you take. Or one that increases the positive effect of other potions at the expense of additional toxicity. I'd especially like one that makes other potions last longer.

Kibou: Thanks for all the positive response! :)

A little problem I shouldn't miss to mention would be balancing in regard to the alchemy tree, since it has skills with similar effect: "specialization: potions", that enhances potion duration, and "Catalysis", that enhances positive and dimished negative potion effects. Supplemental potions should neither entirely replace this skills and make them redundant, nor should they, when used in combination, make Geralt over-the-top powerful.
Guess a skill that would go well with supplemental potions is "Taster", that allows drinking of a fourth potion, since they'd allow for more interesting combinations.

archaven: ...or make the animation for drinking potions and meditation faster.

Kibou: Supplemental potions should neither entirely replace this skills and make them redundant.

vAddicatedGamer: Would like this as well. Playing as potion-chugging build you can really feel the (unnecessary) lengthiness of the potion-drinking process.
vAddicatedGamer: You were suggesting a supplemental potion to increase potion length, yet there is already a skill that does that which you mentioned. Isn't that a bit contradictory?