The Tavern

I was thinking That along with Ignite poison, there should be a branch of poisonous spells for every element, giving venom mages a better opportunity to segway into elemental magic.

Befoul Earth-
Poison Magic/Earth Magic

Level 5
Somewhat Loud

Befouls a of strip of land, turning it the ground rancid. area occasionally releases miasma, with affected area and frequency of clouds occuring with increased spellpower. Corpses in the area rot significantly faster, and rotted corpses decompose.
Each casting causes minor sickness.

visually, it'd be a 6x3 plot of land, longways from the caster.
....xxx....
..xxxxxx..
....xxx....

I actually think this is one of the more interesting ways to make poison more...well, interesting. I think the argument people are going to raise is the typical "there's plenty of xp for raising the other skills" one, but this idea seems like fun. Air is probably fine with Meph Cloud, but here's my take on an ice one:

My thoughts:

Frozen Sludge-
Poison Magic/Ice Magic

Level 3
Quiet/Somewhat Loud

Shoots a short-range ball of frozen sludge at an enemy, dealing 10% irresistible damage. Has a chance based on spell damage to burst, dealing 10% damage to all of the surrounding enemies, with a chance to poison all enemies hit.

Well, the argument I'm going to raise is that the devs have said they don't want too many easy sources of miasma (Corpse Rot at least needs corpses), and also that miasma falls under the purview of Necromancy, not Poison.