I wish the poison was a different mechanic as well.
My thoughts:
Each time you are hit (Or maybe wounded) with a poisoned weapon, your poison rating goes up. (starts at zero, 1st hit=1, 6 hits=6max)
At the end of any phase in which you became poisoned and at the beginning of each hero phase, you roll a die and if you roll over your poison rating you reduce your poison rating by 1 and take no damage. If you roll below or equal to your poison rating then you take 1 mortal wound and reduce your poisoned rating by 1.
Healing reduces poison rating by the amount healed.
OR
Each time you hit with a poisoned weapon (or wounded), choose and apply of the following debuffs that lasts until your next hero phase:
-1 to hit rolls for poisoned unit, +1 to hit rolls against poisoned unit, -1 to wound rolls for poisoned unit, +1 to wound rolls against poisoned unit, -2 to bravery, 1/2 movement, -1 to saves, Can't move, Can't pile in, Fight last, Can't be chosen to fight, Can't Fire Ranged Weapons, Can't cast spells, Can't cast prayers, Can't give or receive commands. (Just a list of ideas)
Healing removes 1 debuff per amount healed.
OR
Poison could just make it a lot easier for a weapon to wound. Like all poisoned weapons wound on 2+
OR
Poison could make the damage of weapons higher than their un-poisoned counterparts. Poisoned weapons do atleast 2 damage, but often 3-4. (High damage, but low or moderate rend)
OR
In the movement phase, Instead of moving, a unit could apply poison to their weapons, improving the rend or damage of the weapon by 1 or x. x=the poison rating of the weapon. OR this is what could make the weapon do mortal wounds.
OR
At the beginning of the shooting or combat phase, a unit can apply poison to their weapons, improving the rend or damage of the chosen weapon by 1 or x where x=the poison rating of the weapon.
I dont know if any of these are good or balanced...but just ideas.