I had a good poison rework idea earlier- Four discrete levels of stacking! Poison could work as it does normally for the first level, but if an already poisoned creature was again poisoned, the poison skull would turn yellow, and the troop would always take 1 damage per turn until cured. If poisoned again, it would turn red, dealing 2 damage per turn, and after 1 more poisoning, it’d turn black- 3 damage per turn, and unable to be cleansed.
Some troops could be reworked to give 2 stacks of poison, but that would be restricted to epic and above troops. This would also make the Venomous trait not completely useless.