Has anyone done a sort of poison overhaul? I want poisons that slow enemies down while killing them and things like that. Maybe some effects that aren't in the vanilla game.

The closest thing that I can recall is http://www.tesnexus.com/downloads/file.php?id=25226, which adds a lot of new functions and features to Alchemy.

http://www.tesnexus.com/downloads/file.php?id=10294 seems to be relatively simple and makes potions and poisons more powerful and potent.

I'd pay particular attention to these mods by Haama, Master of Alchemy : http://www.tesnexus.com/downloads/file.php?id=11183 add the effects of one poison to another. http://www.tesnexus.com/downloads/file.php?id=11185 "allows you to remove beneficial effects from the potion(s) of your choice, turning it into an effective poison".