Idea that spontaneously popped into my head; alchemy ingredients are essentially what remain of spell creation in Skyrim, as each one is really four magic effects, if two match, prestobingoitisinthefinalproduct. This leaves, actually, a lot of room for playing around with; things like limited duration reflect spell effects, perhaps poisons which make the target immolate everyone around them, perhaps a potion of invisibility which constantly recasts every few seconds, a potion that fully restores all traits but paralyzes -you- for a few seconds and makes the outside world seem to speed up.

The chief problem would be: where are the ingredients going to come from? New ones in theory would be better, sure, but they'd require a lot of time placing and leave room for a lot of issues, because players have to be able to go back and harvest them.

Perhaps you could have the ingredients be new parts of plants (seed, stamen, parts of the stem, etc.) in vanilla Skyrim. That way you only need to edit LeveledItem lists (if I've understood the system correctly) instead of having to place the ingredients around Skyrim by hand.

What about being able to create a poison that makes a character drowsy or experience muscle weakness and forces them into the bleedout animation for a short duration?

Ooh, that would be interesting. I think there's also a lot of room for poisons that can only be applied out of combat.

I like the 'new parts of plants' suggestion; it's much more elegant than the one I came up with, which I might use for some other elements. (Namely, use some sort of spell to 'prepare' ingredients and unlock a new set of traits on them.) Lets us keep things rare so they don't supercede the base system.

the new parts of plants idea is great. you could also add new ingredients from vanilla creatures such as

troll blood
wolf brain

etc....